Prime Minister Gordon Brown vowed to maintain "pressure for change" in Burma, ahead of demonstrations against suppressed protests in the country.
Monks have been arrested after protests in Burma
He said the international community would work to move towards democracy.
Campaigners gathered in over 30 cities worldwide to back Buddhist monks who want military rule to end. A London rally was staged in Trafalgar Square.
At least 10 people were killed in the military response to demos. Dissidents estimate that the death toll is higher.
'Process of reconciliation'
The prime minister said: "We will not tolerate the abuses that have taken place. And I want all the other leaders of the world to work with us, to achieve the progress that all of you people want to achieve in Burma - an end to abuse of human rights.
"We want the violence to stop against the people of Burma, and we want to move forward with a process of democracy and reconciliation."
Mr Brown, who welcomed demonstrators ahead of the London event, said he wanted the United Nations Security Council to oversee a "process of reconciliation" in Burma, and called on the European Union to impose further sanctions on the country's regime.
The prime minister said an extra £1m would be found for emergency humanitarian aid for the Burmese people.
The central London demonstration involved a march from the Tate Britain art gallery to a rally in Trafalgar Square.
Some monks threw flower petals into the Thames as they crossed Westminster Bridge in a show of support for the protesters in Burma.
MEP Glenys Kinnock told the demonstrators: "We say to that junta in Burma we are still watching you, do not think our eyes will leave you for one second. We are watching you.
"We are telling the people of Burma that we will not ever waver from that solidarity with you that we are showing here today."
Events also took place in Edinburgh and Glasgow, including a candle-lit vigil at a tree planted in honour of Burmese pro-democracy leader Aung San Suu Kyi in Edinburgh in 2005.
Demonstrations were scheduled to take place at noon local time in Austria, Australia, Belgium, Canada, France, India, Ireland, New Zealand, Norway, South Korea, Spain, Thailand and the US.
Red headbands were to be tied onto government buildings, religious shrines or key landmarks.
In the UK the Trades Union Congress called on European Union leaders to make it illegal for companies in Europe to invest or trade in goods from Burma.
General secretary Brendan Barber said it was time to "ratchet up the pressure on Burma's brutal dictatorship".
The demonstrations marked 20 days since monks first took to the streets to lead protests against the military rulers.
BBC sources in Burma say as many as 10,000 people - many of them monks - were rounded up for interrogation following the protests.
The Burmese ambassador to the UN, Kyaw Tint Swe, said 2,095 people had now been released including 722 monks.
UN envoy Ibrahim Gambari has briefed the UN Security Council after a four-day visit to Burma, where he met senior generals and opposition leader Aung San Suu Kyi.
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