A central figure in Russia's revolution of the past decade who was once seen as a possible president of the country, Anatoly Sobchak died of a heart attack aged 62 while drumming up electoral support for his former protege and now acting president, Vladimir Putin.

Sobchak, a former law professor and democratic activist, was mayor of St Petersburg from 1991 to 1996, the city he himself renamed from the former Leningrad.

He called himself a "radical realist" and said he was inspired by the speeches of Martin Luther King. He was most renowned for his starring roll in the perstroika drama under Mikhail Gorbachev at the end of the 1980s, for his cool and courageous defiance of the old guard putschists in the summer of 1991 and for the early promise of his administration in Leningrad.

A gifted public speaker in a political culture better known for long-winded bores, Sobchak, handsome and nattily dressed, flourished in the democratic upheavals of 1989. He was particularly unusual in that he had never been a member of the Communist party. He finally joined it in the dying days of the Soviet Union (he left it two years later accusing it of a failure "to offer the country a real program for moving to a new society") and sat in the Congress of People's Deputies, the last Soviet parliament, elected in 1989.

It was the high point of the Gorbachev era. The nation was glued to the television set, absorbed in the thrilling debates that dominated the congress. Sobchak instantly became a star, a Gorbachev ally and one of the congress' most effective performers, his legal training helping him to cut through the weighty parliamentary procedure.

A canny politician, he also became close to Boris Yeltsin, running the Russian presidential election campaign for Yeltsin in Leningrad in 1991 and delivering the vote. By then, he was the second most popular politician in Russia and a potential president.

He instinctively rushed to Yeltsin's side in August 1991 in Moscow, supporting the Russian president in his finest hour against the Communist putschists. Sobchak then risked arrest by flying home to Leningrad and confidently confronting the regional military commander, who was poised to order the troops out into the streets in support of the coup.

In Leningrad, the troops stayed in barracks. Sobchak went on television that evening to denounce the plotters. The next day he appeared before a huge pro-democracy demonstration in the city, perhaps the biggest since the 1917 revolution, and was acclaimed as a hero.

Sobchak was born at the height of Stalin's Great Terror in the Siberian railway town of Chita, near the Chinese border. His grandfather was an old Bolshevik who died after being incarcerated by Stalin. Sobchak was good at school and got a place to study law at the University of Leningrad, the city he would later make his home and power base. But he notably eschewed any Communist party career.

On graduation, he practiced law in Stavropol, Gorbachev's home turf in southern Russia, before taking up a law teaching career in Leningrad in the 1970s. An early sign of his radicalism was a pro-capitalist doctoral work submitted – and rejected – in 1973. He specialized in economics law, later writing several books on the subject. By 1981, he had been appointed Leningrad University's first professor of economics law.

After starring in the Congress in Moscow in 1989, Sobchak moved home to head the city council and in 1991 was elected mayor following a month of paralysis in the Leningrad parliament. He held the post until 1996, when he narrowly lost his bid for a second term amid mudslinging and allegations of sleaze and nepotism. (Declaring his loyalty to Sobchak, Putin also left St. Petersburg politics when his boss lost in 1996 and moved to Moscow into the Kremlin administration of Boris Yeltsin.)

In November 1997, Sobchak fled to France fearing arrest on corruption charges. He had been summoned to give evidence over a municipal scandal for allegedly arranging to have his flat upgraded and for accepting another apartment as a gift for his niece. He returned home from France to practice law last summer after the corruption case against him was closed, a move credited to Putin, who was then head of the FSB, the successor to the KGB.

Since Putin succeeded Yeltsin on New Year's Eve, Sobchak had been constant in his support for the new leader, stressing Putin's democratic convictions amid growing fears of the leader's authoritarian inclinations.

Sobchak died in Kaliningrad while campaigning as Putin's envoy, seeking regional support for the presidential election on March 26.

Sobchak's autobiography "For a New Russia" (1992) is an insider's account of the struggles within the fledgling democratic movement to break the power of the Communist party.

He leaves his wife and two daughters.