The summits mark the city's 300th anniversary

UK Prime Minister Tony Blair flew in from Poland, where he urged Europe and the US to heal the divide caused by the Iraq war and called on Poles to vote to join the European Union.

Russia is holding two separate meetings with the former Soviet republics and the European Union.

On Sunday the Russian and US leaders will meet for the first time since Moscow publicly opposed the war in Iraq.

US President George W Bush has been speaking of his good relations with President Vladimir Putin, and officials say the talks will offer the chance to heal the rift over Iraq.

In an interview with Russian state television, Mr Bush strongly defended his decision to invade Iraq and dismissed speculation that Washington was considering attacking Iran or Syria.

City gets makeover

Russia has spent $1.5bn to give St Petersburg a face-lift, as President Putin - one of the city's most famous sons - prepares to host what correspondents say is one of the most important summits of his career.

In pictures: St Petersburg at 300 Analysis: Bush's grand tour

The city - founded by tsar Peter the Great - has been celebrating its anniversary all week with open-air concerts, laser shows and grand regattas on the Neva River.

But the BBC's Steve Rosenberg, in St Petersburg, says there was an inauspicious wet start to the celebrations, as 40 "rather confused" leaders cowered under umbrellas on their way to a service at St Isaac's Cathedral.

Between the summits, Mr Putin is due to take on the role of tour guide, taking world leaders around the palaces and museums of Russia's most European city.

'A crucial time'

Accompanied by his wife Cherie, Mr Blair is scheduled to meet Mr Putin on Friday evening and Saturday, which also sees the staging of the formal Russia-EU summit.

In a speech at Warsaw's Royal Castle earlier on Friday, Mr Blair warned that now was "a crucial time" for the international community.

Iran will be a key issue during Sunday's talks

Earlier, leaders of the former Soviet republics at their summit in St Petersburg approved the lifting of the UN sanctions on Iraq.

Meeting as the Commonwealth of Independent States, the leaders spoke out in support of enhancing the role of the United Nations in the post-war reconstruction of Iraq.

The BBC's Nikolai Gorshkov, in St Petersburg, says the CIS backed Russian aims to mend relations with the West over Iraq, with all members hoping to see a stable regime in the region close to their southern borders - but they mostly discussed their own economic problems.

They gathered aboard the Silver Whisper cruise liner moored for the occasion on the Neva River.

Of particular interest to CIS members will be Russia's summit with the EU which will include the leaders of 10 future EU members, most of them former Soviet allies, our correspondent adds.

Japanese Prime Minister Junichiro Koizumi and Mr Putin - who also met on Friday - agreed to boost joint work on energy projects, and reaffirmed the need to sign a post-World War II peace treaty.

St Petersburg is the first leg of Mr Koizumi's seven-day trip to Europe which started on Thursday.

Thorny issue

Mr Putin played a full role in the start of his native city's celebrations, watching children dance on a newly-renovated St Petersburg square, and praising the city's founder Peter the Great in a speech in front of one of the city's ornate palaces.

Foreign dignitaries continued to stream into the city throughout the day, some bearing gifts in honour of the anniversary.

The Canadian Prime Minister Jean Chretien unveiled a monument to Canadian poet Emile Nelligan - a gift to the city from Quebec.

German Chancellor Gerhard Schroeder presented a bronze sculpture inspired by Russian writer Nikolai Gogol.

Analysts say Sunday's US-Russian summit is not expected to spawn any key developments, but it could offer the chance for reconciliation after the Iraq war.

They say Mr Putin and Mr Bush could use the talks to reaffirm their intentions to jointly face common threats, including terrorism and proliferation of weapons of mass destruction.

But one of the main thorny issues in the bilateral relations remains Iran.

Despite strong US objections, Russia is continuing to co-operate with Iran in building a nuclear plant in Bushehr.