After the revolutionary events of 1917-1919, the city's population declined, and by 1920 it was only 722,000 people.
About 300,000 people were relocated from the working-class suburbs to the Central regions. In 1919, a Council was established to
settle the plan of Petrograd and its suburbs. In 1923, housing construction began in the city (Zhilmassivov). In the 1930s,
Elagin and Krestovsky Islands were improved with access to the Gulf of Finland. On September 23, 1924, the second largest flood
in the city's history occurred, with water rising 380 cm above the ordinary. In 1933, the construction of The Kirov stadium
began on the Western tip of Krestovsky island.
In 1931, the two largest cities of the RSFSR: Moscow (June 16) and Leningrad (December 3) were separated into separate administrative units-cities of Republican subordination of the RSFSR. In 1935-1937, the first General plan of Leningrad was developed, which provided for development in the southern direction with borders in the area of the Pulkovo heights. The center was to be the square at the intersection of International Avenue and the Central arc highway (now Moscow square) with the house of Soviets and other administrative buildings. By 1939, a new version of the master plan was developed, which was never approved, but in accordance with it, construction of residential quarters began on Malaya Okhta, Ivanovskaya street, Avtova, and International Avenue.
Palaces of culture were built throughout the city, and by the mid-1930s they were available in all industrial areas. In 1932, the first Leningrad Pulkovo airport was opened. 1 December 1934 murder victim was the first Secretary of the Leningrad regional and city CPSU, a member of the Politburo of the CPSU S. M. Kirov. This event will mark the beginning of the "Kirov stream" and the Great terror, replaced by A. A. Zhdanov.
The heroism and steadfastness of the Leningraders were shown during the great Patriotic war. On September 8, 1941, the enemy reached lake Ladoga, captured Shlisselburg, took control of the source of the Neva river, and blocked Leningrad from land. This day is considered to be the beginning of the blockade of the city by the troops of Nazi Germany from the South and Finland from the North. For almost 900 days and nights, under conditions of complete blockade, the residents not only held the city, but also provided great assistance to the front. According to various sources, between 600,000 and 1.5 million people died during the years of the blockade. So, at the Nuremberg trial, the number of 632 thousand people appeared.
Only 3 % of them were killed by bombing and shelling; the remaining 97 % died of starvation. On the other hand, in the encyclopedia "the Great Patriotic war of 1941-1945", published in the military publishing house of the Ministry of defense of the Russian Federation in 2011-2015, these figures are considered significantly underestimated, since they were "not taken into account unidentified blockade runners who died in the city, and Leningrad residents who died of starvation in the process of evacuation". As a result of the counter-offensive of the Leningrad and Volkhov fronts on January 18, 1943, the blockade ring was broken, but only on January 27, 1944, the blockade was completely lifted. After its removal, only 560,000 residents remained in Leningrad.
Immediately after these events, the restoration of the city began. In September 1945, the school year began and the concert season opened in the Great hall of the Philharmonic. In 1950, the Kirov stadium came into operation. In 1951, a new General plan for the development of Leningrad was adopted, according to which it was proposed to develop the city's territory around the historical center in all directions approximately equally. In the 1950s, new architectural ensembles were created: Lenin square, Kalinin square, and Komsomolskaya square; Moscow Avenue, Engels Avenue, Stachek Avenue, Sredneokhtinsky Avenue, and Primorsky Avenue have acquired a complete architectural appearance. In 1951, the first air terminal in Pulkovo was launched (a new building was built in 1973).
On November 15, 1955, the first stage of the Leningrad metro was put into operation. In 1957, the tradition of the midday firing of a cannon from the Naryshkin Bastion of the Peter and Paul fortress (interrupted in 1934) was resumed, and the country's first official Eternal flame was lit on the field of Mars. In the same year, from the shipyards of The shipyard named after A. Marty launched the world's first nuclear-powered icebreaker, the Lenin, and in 1962 the Kirov plant started producing Kirovets tractors. In 1960, a memorial to the victims of the blockade was opened at the Piskarevsky cemetery and the construction of the new building of the Finnish railway station was completed. In 1962, a 316-meter TV tower was built on Aptekarsky island and a new TV center was built.
With the construction of several large house-building complexes in the early 1960s, mass construction of the city began with Khrushchev houses, and since the 1970s with "ship houses". On may 6, 1965, the city of Leningrad was awarded the title hero City (it was first named In the order of the Supreme Commander Of the armed Forces of the USSR, I. V. Stalin, dated may 1, 1945). In 1966, the last Soviet General plan of Leningrad was approved. In accordance with it, in the 1960s, mass construction of the Western part of Vasilievsky island began on alluvial territories, along novoizmaylovsky Avenue, Yuri Gagarin Avenue, and Cosmonauts Avenue. Kupchino, Avtovo, Ulyanka, Dacha, Grazhdanka, Polyustrovo, and Okhta are becoming new large residential areas. For the anniversary of October, in 1967, the Yubileyny sports Palace and the Oktyabrsky Grand concert hall were built.
In the 1970s, Uritsk, Sosnovaya Polyana, Vesely Settlement, the area North of Murinsky brook, the territory of the former Commandant's airfield, the southern part of kupchin, Shuvalovo and Ozerki, South-West, Rzhevka and Gunpowder were built up. In 1979, construction of a dam began in the Gulf of Finland to protect the city from flooding. In 1982, a Marine station was built in the Harbor on Vasilievsky island. In 1988, the 5-millionth resident of Leningrad was born. In 1990, the historical center of the city was included in the list of UNESCO world heritage sites.