(File image) Russian President Vladimir Putin

London - Black-belted Vladimir Putin locked horns on Thursday with British leader David Cameron over Syria before heading for an afternoon of judo diplomacy at the Olympics where he may face questions over a crackdown on Kremlin opponents.

Prime Minister Cameron tried to push the former KGB spy to take a tougher line on Syria, Russia's firmest foothold in the Middle East, and stop blocking Western-backed resolutions aimed at stepping up pressure on President Bashar al-Assad.

But after 45 minutes of talks in Downing Street, for which Putin put in an unusually punctual appearance, Cameron and Putin said Russia and Britain still differed over Syria.

“I look forward to taking the president to the judo but note that we will be spectators, not participants,” Cameron told reporters before the two men travelled in separate cars to the Olympics.

In a stark illustration of the still frosty ties between Britain and Russia, Putin used his stretched, Russian-number-plated black Mercedes limousine while Cameron used his armoured grey Jaguar to leave Downing Street.

The two leaders entered the Olympic judo arena together just seconds after British fighter Gemma Gibbons pulled off a shock win over the world champion to get to the final of one of the women's competitions. The two sat down and were seen talking animatedly with each other, both occasionally gesticulating.

Russia's 59-year-old president will also probably face questions about an intensifying Kremlin campaign to silence dissent after members of a women's punk band went on trial and a prominent opposition blogger was charged with embezzlement.

As Putin entered the prime minister's office in central London, one protester screamed: “Free Pussy Riot”, which echoed across Downing Street, in reference to the band who sang out an anti-Putin punk prayer in Moscow's main cathedral.

A one-time judo champion in his native city of St Petersburg, Putin is likely to be at his combative best during his first visit to Britain in years.

For the Kremlin leader, who revels in his hard-man image, the sight of judokas body-slamming each other on the Olympic mat will offer a powerful backdrop to his talks with Cameron.

Their body language and dynamics will be watched closely for any clues on the progress of the talks.

London Mayor Boris Johnson, known for his wisecracks, said he hoped Putin would strip off to take part in the judo.

“Oh, I hope he will take part. What is he, a dab (hand), I think that's what we want to see, stripped to the waist. We want the politicians Olympics, that's what we want,” Johnson told reporters.

Russia has faced growing Western criticism of its position on Syria, with the United States and Britain demanding Moscow drop its support for Assad.

Western powers believe that ousting Assad is the only way to end the bloodshed in Syria. Russia, on the other hand, provides arms to Damascus and has blocked three Western resolutions calling for an increase in pressure on Assad.

Apart from Syria, talks will probably touch on Russia's tense political situation where critics say authorities seek to silence Putin's opponents through unfair accusations and trials.

In a letter in the Times newspaper, a dozen leading rock musicians including Jarvis Cocker urged Putin to give a fair trial to women's rock band Pussy Riot, whose members face up to seven years in jail for protesting inside the Moscow cathedral.

Officially, both sides have said talks would focus on trade.

“The sides will discuss what can be done to boost trade. Apart from this, in the course of the talks, the Syrian situation will be discussed in great detail as well as other regional problems,” a Kremlin spokesman said.

“We see it as another possibility ... to explain our arguments, the Russian Federation's clear, consistent and transparent position.”

Thursday's judo bouts include the men's 100kg and women's 78kg competitions, with the two finals around 1500 GMT. Russia's Tagir Khaibulaev and Vera Moskalyuk as well as Britain's James Austin are expected to compete in Thursday match ups.

Russia and South Korea are currently joint top of the judo medals table with 2 gold and a bronze. Russia hadn't won a judo gold since the break-up of the Soviet Union so the London Games are by far their most successful games in judo.

Japan are traditionally the strongest country but in the men's field the Russians are doing far better.

Putin, whose testosterone-fuelled appearances have earned him the nickname “alpha-dog” in U.S. diplomatic cables, is in London - home to many influential Russians and political exiles - on a private visit at Cameron's invitation.

But diplomatic efforts are complicated by Russia's difficult relations with Britain itself, ranging from espionage to human rights to the 2006 death from radiation poisoning in London of Kremlin critic Alexander Litvinenko.

“He (Putin) is not welcome in London, neither by Russians who live here or Londoners themselves,” Litvinenko's widow, Marina, who lives in Britain, told Reuters.

“It will not be a comfortable visit for Putin. A lot of uneasy questions will be raised.”

Observers said Putin's show of strength in London could be in part aimed at the audience at home, where Putin has hardened his anti-Western rhetoric in response to a wave of anti-government protests this year.

“From his choice seat at the Olympic Games Mr Putin will be closely watching the international reaction to his latest crackdown,” Garry Kasparov, a liberal opposition leader and former world chess champion, wrote in the Times newspaper.

Russian dissidents in London held a small protest outside Russia's embassy ahead of the visit, shouting “Shame on Putin” and “Russia will be free”.

Media mogul Rupert Murdoch, on his official Twitter account, said: “Interesting today to see if any decent protests against Putin his phony trials of dissenters. Russia even less free speech than here.” - Reuters