COPENHAGEN (AFP) - Participants at the UN climate summit waited outside in the cold for hours on Monday to enter the jam-packed Bella conference centre, as Copenhagen was to tighten security ahead of the visit of hundreds of political leaders.

Hundreds of newly arrived delegates, observers and reporters flocking to the Danish capital for the last five days of the climate summit were forced to wait for up to six hours, some of them giving up, to get into the conference centre and police temporarily blocked off the Bella centre subway stop.

The tension amongst Danish police and organisers was to be put to another test as security tightened ahead of the arrival Friday of US President Barack Obama and heads of state and government from China, France, Germany and Russia to take part in the talks.

More than 45,000 people, amongst which 22,000 participants from non-governmental organisations (NGOs), registered to enter the conference center, which has a capacity of 15,000, UN official Axel Wuestenhagen said during a press briefing.

On Monday, organisers announced drastic measures to remedy the conference center's busted capacity, most of which were to affect NGOs.

Their number of accreditations was to be drastically cut down to 7,000 for Tuesday and Wednesday, and to 1,000 for Thursday.

And when the high-level talks will take place on Friday, NGOs will have to scramble to reduce their numbers to a meager 90 people, a shock to many organisations puzzled by the new quotas.

"They're taking in leaders and putting the people out," Canadian Ricken Patel, executive director of Avaaz, told AFP.

"We can see from these negotiations that we don't fix climate unless our leaders feel the pressure ... I'm worried that they're taking the pressure off exactly at the time when we need it," he said, explaining his NGO was going to have to cut its 200 participants to 39 as of Tuesday.

Jeremy Osbourn, responsible of logistics at NGO 350.org, told AFP he had to ask 300 of his organisation's 350 participants to leave the conference.

"It's incredibly frustrating. They're squeezing us out of here, that's hard. But in a way the bureaucracy of it all starts to focus us back on the movement that is outside these walls," he said.

"While we understand considerations of safety and security in the UN Convention Centre... Oxfam is concerned that the dramatic nose dive in the number of passes for observers will decrease the openness and accountability of these talks," Oxfam's executive director Jeremy Hobbs told AFP.

Many NGO representatives told AFP they would plan more actions outside the all but locked-down conference centre in the wake of the reduced number of accreditations.

The influx of delegates, lobbyists, reporters and activists already caused headaches to authorities in Copenhagen, and the Danish police mobilised more than half of its troops for the conference.

Streets in the normally calm capital were shut down amidst almost daily protests, and hotels were full to the point that many delegates had to commute from neighbouring Sweden or stay more than a hundred kilometers (60 miles) from Copenhagen.

Many activists were staying in gyms, at people's houses or in warehouses rented by NGOs. A cruise ship built for 1,400 passengers was rented in the Copenhagen harbour by a Norwegian group, with hundreds of activists on board.