LONDON — The Russian opposition figure Vladimir L. Ashurkov breathed a sigh of relief when he fled Moscow for London in 2014. After months of being followed by the Kremlin’s intelligence agents to meetings, culminating in a televised raid of his apartment, he finally let his guard down, disappearing into the elegant, polyglot streets of Kensington.
Six months passed before he realized that he was still being followed.
An old friend returned from a trip to Russia with unnerving news: In Moscow, security officials had asked detailed questions about a private conversation he had with Mr. Ashurkov in a London cafe. As he built his life in London, Mr. Ashurkov learned to look for Russian agents reflexively — men in dark suits sitting alone at émigré gatherings, dinner-party acquaintances rumored to be informants.
“You can’t do much about it,” he said. “Even after you escape from Moscow to London, you know they have long hands.”
Russia now has more intelligence agents deployed in London than at the height of the Cold War, former British intelligence officials have said. They serve a variety of functions, including building contacts among British politicians. But the most important task is to keep an eye on the hundreds of heavyweight Russians — those aligned with President Vladimir V. Putin, and those arrayed against him — who have built lives in Britain, attracted by its property market and banking system.
The poisoning last week of Sergei V. Skripal, a retired Russian double agent, and his daughter has put pressure on the British government to rein them in.
The British authorities once devoted abundant resources to tracking the movement of Soviet agents here. But in recent years terrorist threats have become the clear priority, and MI5 has fewer resources to keep pace with Russia’s expanding operations, said John Bayliss, who retired from the Government Communications Headquarters, Britain’s electronic intelligence agency, in 2010 and now lectures on security threats.
“I think it’s sort of accepted that there are more spies in London now than there were at the height of the Cold War,” he said. “In the Cold War, it was quite difficult for Russians to move around the country, they were restricted outside London. But now they’ve pretty much got free movement, they can go anywhere. We haven’t got enough people to follow everybody all the time.”
London is also a base for commercial intelligence-gathering firms, like the one headed by the former MI6 agent Christopher Steele, who built a dossier on President Trump’s links with Mr. Putin. The Russian government is keenly interested in these efforts, and their sources.
As a young K.G.B. officer, Mr. Putin was first assigned to a station in the East German city of Dresden, which dispatched spies to steal technological secrets and compromise and recruit influential figures, in both West and East Germany. As Russia’s leader, he has expanded foreign intelligence networks so that they “reached or surpassed Cold War levels,” said Mark Galeotti, a Russia expert at the Institute of International Relations in Prague.
“There is a sense that Russia is geopolitically in competition with the West,” he said. “In these current circumstances, spies are relatively cheap and relatively effective. This is the way Putin runs his state.”
Over the last 10 years, Britain has granted political asylum to a parade of Mr. Putin’s critics, big and small, who have blended seamlessly into “Londongrad.” It is a place where, as one denizen explained, a bureaucrat on vacation could dine cordially with a dissident novelist who speaks at anti-Putin rallies.
A former British intelligence official, speaking on the condition of anonymity in line with protocols, described it as “a lot of Putin’s friends, and former friends, and enemies and allies, all swirling around together in this moneyed scene.”
“And of course half of them send their kids to British public schools,” the former official added, using the British term for private schools.
In interviews, prominent Russians here described a growing awareness that they were under close watch by the Kremlin.
Yevgeny Chichvarkin, a mobile-phone tycoon who complained publicly about official corruption, fled Moscow for London in 2009 and was later accused of kidnapping and blackmail. Mr. Chichvarkin, who met for an interview in a fashionably moth-eaten pink cardigan and gaudy pantaloons, now owns Hedonism, a Mayfair wine shop where one bottle of vintage cognac is priced at $ 340,000.
Mr. Chichvarkin said he had realized that he was under surveillance shortly after moving here, when he observed a group of two or three men standing for hours around 100 yards from his front door. Peering at them more closely, he saw that they were passing the time by peeling and eating sunflower seeds, a habit common among men from the Russian countryside.
“Stirlitz had arrived,” he said, referring to an iconic K.G.B. agent, the hero of a 1973 television series set in Nazi Germany, “Seventeen Moments of Spring.”
Still, Mr. Chichvarkin said he felt far safer in London than in Moscow, particularly after the 2015 assassination of the prominent opposition leader Boris Y. Nemtsov. He said that Russian security personnel were far more constrained on British soil — “In Moscow, they can use guns,” he said — and that he had been deeply impressed by British policing and rule of law.
“I think they do more than all the other countries in the European part of the world,” he said.
Mikhail B. Khodorkovsky, a Russian tycoon who served a decade in a Russian prison and received asylum here last year, wanders London without bodyguards, and can be seen in line for takeout coffee at the sandwich chain Pret a Manger. For his part, Mr. Ashurkov, who was granted political asylum in March 2015, learned to relax in London after what he described as “a period of paranoia.”
“I know that Russian security services are capable of assassinations in London or any part of the world if a decision is made in Moscow, but you cannot think about it all the time,” he said.
Other opposition figures, however, say they are constantly on alert.
Mr. Skripal is the second former Russian agent to be poisoned on British soil, after the 2006 killing of Alexander V. Litvinenko, which, a British inquiry concluded nearly 10 years later, was in all likelihood ordered by the Kremlin. The British government took modest countermeasures, and the two men accused of the killing remain at large in Russia.
Bill Browder, a wealthy investor who has led international campaigns to impose sanctions on Mr. Putin and his associates over corruption and human rights violations, says the poisoning of Mr. Skripal means that people like him are at greater risk.
“The British government has created a dangerous situation by not creating consequences for Litvinenko,” said Mr. Browder, who has lived in London for three decades and has British citizenship. “If there is no reaction for Skripal after Litvinenko, then there is a high probability I will be killed next.”
For years, the fulcrum of London’s anti-Putin opposition was Boris A. Berezovsky, a billionaire who broke bitterly with Mr. Putin and received political asylum in 2003. Lord Timothy Bell, who was close to Mr. Berezovsky, said he had often sat in public places as his friend pointed out the agents following him.
“They weren’t with anybody,” he said. “They weren’t sitting sociably. They were staring. It was unnerving. You got used to it after a while.”
Mr. Berezovsky employed high-priced security services, staffed by former officers in the Israeli military or French Foreign Legion. But the cost became unmanageable as his fortune diminished, and the number gradually dropped from six to four to three to two, Mr. Bell said. Only one bodyguard was guarding him in 2013, when he was found dead in the locked bathroom of a manor house.
“I suppose it was not worth paying for, since it didn’t protect him in the end,” Mr. Bell said.
The coroner in the case said, the next year, that it was impossible to say whether Mr. Berezovsky had killed himself or been murdered. Mr. Chichvarkin, another friend, recalled that Mr. Berezovsky had regarded his security detail with a degree of fatalism.
“He used to say, ‘He’s not a bodyguard, he’s a witness,’” he said.