When Vladimir Putin took over as acting president of Russia on the last day of the 20th Century, the former spy was an enigma to many. In History looks at how the surprise leader survived a tough childhood to rise to power in the Kremlin.

Russian president Boris Yeltsin made the shock announcement on 31 December 1999 that he was resigning, telling television viewers: "Russia must enter the new millennium with new politicians, new faces, new intelligent, strong and energetic people." Amid widespread corruption and huge political and social problems, Yeltsin's presidency had become increasingly unpopular and unpredictable. While he played a key role in bringing down the Soviet Union in 1991, his time in office had been a traumatic period for Russia as it transformed from a communist state-run economy to a free-market one.

At midnight, Yeltsin's heir apparent Vladimir Putin –  a new face for the new millennium – made his first televised address as acting president. "There will be no power vacuum," he promised. There was a warning, too. "Any attempt to exceed the limits of law and the Russian constitution will be decisively crushed," he said. The lean, fit and sober Putin proved popular in a country used to the erratic behaviour of Yeltsin, who was so boozy and unhealthy that it was sometimes a news story when he managed just to make it into the office.

When Putin became prime minister in August 1999, he was an ex-KGB man plucked from relative obscurity. By the end of the year when he took over as acting president, he had won popularity for his tough line on the war in the breakaway republic of Chechnya. When elections were held in March 2000, Putin was confirmed as president after securing almost 53% of the vote in the first round. Polls suggested that most Russians wanted economic stability above all else. Putin's basic message to voters was that he would make Russia strong again.

The new leader of the world's largest country had risen to the top while leaving few traces. It was clear the 47-year-old was a man who liked to look and talk tough – a judo black belt who would make pronouncements such as calling lawbreakers "rats who should be squashed". But what was he really like? 

Putin grew up in St Petersburg, known then as Leningrad. Founded by Tsar Peter the Great, it was a city full of western influences but also echoes of Russia's grand imperial past. The BBC spoke in 2001 to Putin's old judo coach, who said that he was a star pupil who had the potential to make the Olympic team. Anatoly Rakhlin explained that Putin was always determined to win, if not by brute force, then by outwitting his opponents: "He could throw with equal skill in both directions, left and right. And his opponents, expecting a throw from the right, wouldn't see the left one coming, so it was pretty tough for his opponents to beat him, because he was constantly kind of tricking them."

Putin was born in 1952, seven years after the end of World War Two, following the siege of Leningrad that killed his elder brother and which his parents barely survived. He was brought up in a crowded communal flat with shared kitchen and bathroom, teeming with rats and cockroaches. He recalled in his autobiography how as a boy he had to fight rats on his staircase. He wrote: "Once I spotted a huge rat and pursued it down the hall until I drove it into a corner. Suddenly it lashed out and threw itself at me. It jumped down the landing and down the stairs."

The tone of his famous cornered rat anecdote becomes more or less aggressive depending on his audience, according to Prof Nina Khrushcheva, the great-granddaughter of former Soviet Premier Nikita Khrushchev. Speaking on the 2023 BBC podcast Putin, she said: "He chooses to tell it all the time to show his modest upbringing and how far he has come and what kind of enemies he had to endure in his lifetime; how he began enduring the lowest form of creatures, then moving up to face all sorts of enemies, foreign and domestic."

Childhood friend Maria Osorina, a psychologist, told the BBC in 2003 that it was "survival of the fittest" in the tough environment they grew up in. "He was small, thin and rather weak, because he was born of such old parents, and so it was very important to him to be strong so that he wouldn't get beaten up," she said. 

She said that the family had strong values of duty, patriotism and loyalty. "His parents loved him very much. He was the centre of their world, the son they'd longed for. But their character was very restrained by nature – they didn't really show their emotions. The father was outwardly very cold, and his mother, too. They wouldn't even consider kissing their son in public – that would never have occurred to them."

Friends and acquaintances remembered the young Putin as clever but self-contained. He was "never the centre of attention", schoolmate Sergei Kudrov told the BBC in 2001. "He preferred to influence events from a distance, a sort of 'grey cardinal', as the saying goes. So different from Boris Yeltsin. Remember how he climbed on a tank and gestured for everyone to follow him? You just couldn't imagine Putin doing that. He is an introvert – a man of deeds, not words." 

He had a romantic desire to become a KGB agent and serve his country incognito – perhaps the perfect job for someone who liked to avoid the limelight. By his own admission, his inspiration was the 1968 Soviet spy film, The Shield and the Sword. It was about a Russian double agent in wartime Germany, stealing documents to sabotage Nazi operations while posing as a chauffeur.

Putin never wavered from his boyhood ambition to become an intelligence officer, right through university and KGB training. When he was 16, he entered the local KGB headquarters and asked for a job. They told him to study law and then wait. Six years later he was recruited by the agency. For more than 16 years, Putin would live the double life of an intelligence agent. When the Berlin Wall fell, he was serving in East Germany. He returned to a Russia where all the old certainties were collapsing. 

In 1991, Putin became deputy to the new mayor of Leningrad, Anatoly Sobchak. When Sobchak was voted out, the Kremlin headhunted Putin. As the Yeltsin administration staggered towards its end, Putin rose stealthily until, in 1999, he was made prime minister. The man from nowhere was suddenly everywhere all at once.

For Putin's old friend Maria Osorina in 2003, his leadership was a breath of fresh air: "I was born in 1950, and since that time we've never had a leader who is pleasant to look at. I didn't like any of them. Putin is the first person to rule Russia since the Revolution whom I really like. He's the first normal person, the first one we're not ashamed of."

Putin has been in power for a quarter of a century, longer than any Kremlin leader since Soviet dictator Joseph Stalin. Now that he is aged 72 and in his fifth term as president, wrote the BBC's Paul Kirby earlier this year, "all semblance of opposition to his rule is gone and there is little to stop him staying on, if he wants, until 2036".