At the beginning of May 1941, so-called
May consultations of Communist Party officials from across the country, who sought to organize the resistance against the occupiers, were held in Zagreb. In June 1941, meeting of the Central Committee of KPJ, on which it was decided to start preparations for the uprising, was also held. May Day proclamation of KPJ and proclamation of KPJ and KPH on the occasion of the
Operation Barbarossa, were restored. In proclamation, all people were invited to the armed resistance, while all patriots were called into a united front for the expulsion of the invaders from the country. KPH remained in Zagreb until February 1942, when it moved to the liberated territory. The resistance movement in Zagreb was led by the Local Committee of the KPJ which had six rayon committees. At the beginning of the occupation, Zagreb KPJ organization had 53 basic organizations with about 500 members, of which 75% were workers. In addition, there was a strong
Youth organisation, the "SKOJ", which, in the first half of 1942, had close to 900 members and about 500 supporters gathered in the boards of the
League of the Young Generation. The association organized youth to fight against the occupiers on the anti-fascist platform and was later core of the
United League of Antifascist Youth of Yugoslavia (USAOH). In October 1941, the Initiative Committee of the
Women's Antifascist Front of Croatia (AFŽ) was established, at the end of 1941, the Central Committee of AFŽ, and in February 1942 Local Board of AFŽ. The most massive underground organization in occupied Zagreb was (lit. "People's help"), whose tasks included developing combat anti-fascist consciousness of the citizens, raising money and materials for the Partisans, assisting the families of soldiers, harboring illegal aliens, distributing illegal press, and transporting people to the liberated territory. This organisation's local committees were divided into six district committees, which in turn had a total of more than 700 members, and about 4,000 helpers and supporters. By 1943,
Narodna pomoć was replaced by the
National Liberation Committees.
Notable sabotages in 1941 Soon after occupation, the resistance movement organized a series of sabotages against the occupying forces. Ustaše tended to respond by carrying out acts of repression against prisoners in the case of anti-fascist sabotage or assassinations of German or Ustaša officials. One of the first manifestations of resistance to the occupation was an action of solidarity of the Croatian youth with Jewish and Serbian youth that was conducted on 26 May 1941 in Zagreb's
Maksimir Stadium when the leader of the
Ustaša Youth gave an inflammatory speech to the students and called for Jews and Serbs to step forward, which was intended to incite a
pogrom. When Jews and Serbs began to step forward, young members of SKOJ followed them, also pulling others with them, so that the pogrom was disabled. The 1977 film
Operation Stadium portrays those events. During June and early July 1941, the resistance sabotaged aerospace batteries in
Prečko and storage of antiaircraft artillery in
Ilica. It also organised the burglary of an ammunition train in the
Zagreb Western railway station, as well as a diversionary attack on the railway lines to
Varaždin,
Sisak and
Dugo Selo and Zagreb Radio station.
Kerestinec prison was a prison for the communists that was formed even during the
government Cvetković–Maček. Upon coming to power, the Ustaše took over control of the prison, which had previously housed many communist intellectuals, among others
August Cesarec,
Božidar Adžija,
Ognjen Prica,
Otokar Keršovani, and . In early July 1941, Communist Party activists , , and others abducted Ustaša police agent Ljudevit Tiljak, a former member of the Party, and killed him after interrogation. The authorities responded by condemning a number of captured Communist Party activists to death. In the night of 13–14 July 1941, KPJ activists organized an escape of detainees from Kerestinec, but because of poor coordination of operations it was not entirely successful, since only 14 of the 91 detainees managed to escape. Ustaše executed survivors in Maksimir and Dotrščina on the next day, including Adžija, Cesarec and Prica. In July, the resistance sabotaged a silk factory, where it burned 50,000 meters of parachute silk. A few days later, SKOJ members set fire to wooden parts of
Maksimir Stadium, whose panels the Ustaše had intended to use for the construction of concentration camp barracks. The first major armed action prepared by the KPJ, SKOJ, students and workers, was the one conducted near
Zagreb Botanical Garden. On 14 July 1941, 12 resistance movement members, led by
Slavko Komar, attacked the members of an Ustaša university company with hand grenades and revolvers, injuring 28 of them. The Ustaša government reacted to this attack with severe reprisals. On 5 August, it was announced that 4 attackers were executed. "In addition, 98 Jews and Communists were sentenced to death by the Senate of the Court Martial as accomplices and intellectual pioneers of the attack." On the following day, 6 August, execution of additional "87 Jews and Communists" as "further accomplices and intellectual originators of the attack " was announced. In September 1941, a group of anti-fascists attacked a bus with German soldiers in Zvonimirova street. In the same month, there was also a sabotage in Cement factory in Podsused. Likewise, members of the resistance in Zagreb organized a successful extraction of
Nada Dimić from the
Sveti Duh Hospital. On 14 September a group of anti-fascists, led by , conducted a long-prepared
Sabotage at the General Post Office in Zagreb, when explosives damaged the automatic exchange device and high-frequency device for long distance communication. On the same day, a detachment of Ustaše was attacked in Vrbanićeva street, and on 30 September, anti-fascists, led by
Ivan Šibl, attacked a group of German airmen in Rusanova street.
Partisan military unit organization By the end of 1941, the main focus of the Zagreb resistance switched from diversions to the recruitment and extraction of volunteers from the city to Partisan-held territories.
Vladimir Popović, Hebrang, Končar and Bakarić likewise left the city the same year. The first
Zagreb-
Sesvete partisan group was formed in
Dubrava in July 1941. In August 1941, a group of fighters from Zagreb traveled to
Žumberak where they formed a partisan unit "Matija Gubec", which had 37 members, but after it was attacked by the much stronger enemy forces it was dissolved. The first partisan unit in the nearby
Hrvatsko Zagorje was formed in March 1942 near the village
Brdovec, roughly northwest of Zagreb. In April 1942, a tenth of fighters was sent from Zagreb to Žumberak where they merged with the First Zagorje unit, after which they grew into the "
Josip Kraš" Battalion. Resistance sabotages also happened during this period. On 12 June 1942 a Wehrmacht gas depot on the Savska Road was set ablaze, destroying around 60,000 kilograms of fuel, several tank engines and machine shops. In the summer of the same year, the first partisan units were formed in
Moslavina (roughly southeast of Zagreb), and in the fall the first partisan unit was formed at
Kalnik (roughly northeast of Zagreb). At the end of November 1942, the "
Pokupsko-Žumberak unit" and the "First Battalion of the Croatian proletarian" merged into the 13th strike brigade "Josip Kraš" (later named "
Rade Končar"). In August 1943, the "
Brothers Radić" Brigade was formed, and in the fall of the same year "
Matija Gubec" Brigade. In July and October 1943, the Partisans entered
Gornje Vrapče neighborhood, in what was then the immediate vicinity of Zagreb, and also penetrated the
Sljeme mountain. In December of the same year, the
Turopolje unit attacked the main aviation depot in Sopnica near
Sesvete while on 19 December the 18th Slavonia unit destroyed the airport at Kurilovec. The 10th Zagreb Corps was formed in January 1944. Upon formation it had around 7,000 fighters. At the end of 1944, the 10th Corps had 9,859 soldiers and officers. It became the carrier of fighting actions in northwestern Croatia and especially in the Zagreb district. In late January 1944, the Youth Brigade "Joza Vlahović" and the "Franjo Ogulinac Seljo" Brigade of the 34th Division were formed. In the fall of the same year, so were the Ljubljana Brigade "
Pavlek Miškina" and the Brigade "Nikola Demonja". ==Allied bombing campaigns==