Hundreds of protesters are marching towards the UN climate summit venue in Copenhagen as ministers begin their discussions there.Activists have been angered by the lack of progress on a new climate deal, and with logistical problems at the summit.
Talks are deadlocked over emission cuts and financial aid for poorer countries.
And as environment ministers and their aides join the talks, the Danish authorities have slashed the number of campaigners allowed into the venue.
Meanwhile, UN Secretary-General Ban Ki-moon has suggested that poor countries may have to give up their hope of getting immediate long-term financial commitments from richer countries.
The amount of aid rich nations will pay poorer ones to combat global warming has been one of the main stumbling blocks at the summit.
In an interview with the UK's Financial Times, Mr Ban said he didn't think the exact amount was vital to the current deal.
"If they are not able to agree this time at Copenhagen, then there needs to be some initial arrangement. This is a time when common sense, compromise and partnership should prevail," he said.
BBC environment correspondent Richard Black, in Copenhagen, says that for the last two mornings, thousands of would-be delegates have queued for hours to gain access to the conference venue - many unsuccessfully.
Marches are being planned during which some protesters are due to attempt an unauthorised entrance.
If they succeed, they will meet up with others who are planning to walk out of the talks and, our correspondent says, it is possible they will be accompanied by delegates from countries that feel their concerns are being marginalised.
Once negotiations do get under way, delegates will find they still have a huge number of fundamental issues to address, 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 2 Celsius or just 1.5 Celsius.
Two years after governments decided to seek a new global climate deal and just three days before it's due to be completed, there's clearly still an immense amount of negotiating left to do, our correspondent adds.
In another development on Wednesday, African nations are expected to reject a deal forged by Ethiopia, France and the UK aimed at breaking the deadlock at the summit, the BBC has learned.
The deal - said to have been organised during Ethiopian President Meles Zenawi's recent visit to Paris - would have raised funds for financing clean development and climate adaptation through "innovative mechanisms" including taxes on air travel and on bank transactions.
'Important moment'
The high-level phase of the talks began on Tuesday, with environment ministers and negotiators from 193 countries struggling to make progress on core issues.
But the White House says President Barack Obama, who will join world leaders in Copenhagen later in the week, is confident of reaching a deal.
"The president believes that we can get... an operational agreement that makes sense in Copenhagen, over the next few days," spokesman Robert Gibbs told a briefing.
UK Prime Minister Gordon Brown, who is already in Copenhagen, told reporters on Tuesday that it was a critical moment.
"This is a very important moment for the world," Mr Brown said.
"It is possible that we will not get an agreement and it is also true that there are many issues to be sorted out. But I am determined... to do everything I can to bring the world together."
More than 120 leaders will formally join the talks on Thursday, aiming to seal an accord by Friday.
