Police have forced back hundreds of protesters who tried to break through a perimeter fence at the UN climate summit venue in Copenhagen.
The Bella Centre, where the conference is taking place, has now been shut off, with no-one allowed to enter or leave.
Activists have been angered by lack of progress on a new climate deal and also by restrictions on access to the talks.
Meanwhile, African countries have softened their demands for climate finance from rich nations.
RICHARD BLACK'S EARTHWATCH
The African group reportedly gave the Ethiopian negotiator a real roasting about this at their routine morning meeting, because the proposal gives ground on some of the African bloc's fundamental points
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Ethiopian Prime Minister Meles Zenawi, speaking on behalf of all African nations, announced the move, which could remove a key obstacle in the talks.
"I know my proposal today will disappoint some Africans," he said.
"My proposal scales back our expectation with respect to the level of funding in return for more reliable funding."
In another development, Danish Environment Minister Connie Hedegaard has resigned as summit president.
She will be replaced by Danish Prime Minister Lars Lokke Rasmussen.
Ms Hedegaard said the move was procedural, adding: "With so many heads of state and government having arrived, it's appropriate that the prime minister of Denmark presides."
However, behind the scenes there are said to be deep tensions between the Danish PM's office and Ms Hedegaard's d
Police detained at least 240 protesters as they marched to the summit across Copenhagen.
From out of the gloom, a few hundred protesters headed towards the UN conference centre. They paused in front of the massed lines of police, blocks of concrete and metal fencing.
Then a female voice urged them to push past the police lines and on to the summit. There was a surge and young people pushed hard against the lines. The crush was obvious. Some protesters squeezed through and climbed onto the roofs of parked vans.
The first one was urged to come down - he refused. His riot policeman nemesis joined him on the roof, and threatened him with his police baton.
The protester ignored him, and a brief game of cat and mouse ensued. Then the policeman hit him hard across the legs, and the protester slid down the windscreen onto the ground.
A couple of others followed suit, but no-one made it past the police ring. The helicopter buzzed overhead. The surge stopped. The stand-off began.
The has been a tense stand-off between protesters and police following earlier clashes.
TV footage showed police using their batons on the crowd and some protesters wiping their eyes after being hit by pepper spray or tear gas. Protesters and police officers were injured in the clashes.
As government ministers from around the world join the talks, Danish officials have cut the number of campaigners allowed in.
Thousands of would-be delegates have queued for hours to gain access to the conference venue - many unsuccessfully.
Those unable to take part on Wednesday included campaign group Friends of the Earth.
It said its delegates had arrived at the centre to find their badges were no longer valid.
Some campaigners said that after marching to the summit they would try to break in.
And sources told the BBC that UK Prime Minister Gordon Brown had been told that he could not leave the Conference Centre in Copenhagen for security reasons. Mr Brown was due to hold a series of bilateral meetings at his hotel close to the conference centre.
Inside the conference, Wednesday's "high-level" session, due to be addressed by prime ministers and other dignitaries, was delayed when several developing countries protested about procedural issues.
China said the process chosen by the Danish hosts "lacked transparency". Others complained that rewritten texts were being pushed through without proper consultation.
* Summit venue is sealed off as police and demonstrators clash outside
* African countries soften their demands for climate finance from rich developed nations
* Danish environment minister Connie Hedegaard resigns as president of the summit to be replaced by Danish PM
* UN chief Ban Ki-moon suggests climate change deal might not include a figure on long-term financial aid for developing countries
The BBC's environment correspondent Richard Black says the summit has been plagued by claims from poorer countries that the Danes have tailored the shape of negotiations to suit the EU's desired outcomes.
Delegates still have a huge number of fundamental issues to address before the summit finishes at the end of the week, our correspondent says.
These include the size of emissions cuts by developed nations, how finance should be raised and disbursed, and most fundamentally, whether a deal here should aim to keep the global temperature rise to 2C or 1.5C.
Draft text released to delegates and obtained by the BBC makes clear that the most important parts of any eventual deal have still to be decided.
Temperature targets are still in the text as alternatives, our correspondent says. Proposed figures for emission cuts by developed nations - apart from the US - range from 15% by the period 2013-2017 to 49% by 2013-2020.
The section on finance consists entirely of paragraphs in square brackets, meaning that none of it has been agreed, our correspondent adds.
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