Security tightens as leaders arrive in Copenhagen

The UN climate conference in Denmark's capital Copenhagen is heading into the final stretch.

On Friday, high-level talks will include more than 100 world leaders. They are expected to sign a political outline of a global warming treaty that will set limits on carbon dioxide pollution.

British Prime Minister Gordon Brown has already arrived in Copenhagen for the Friday conference. He has cautioned against a "failure to act" on climate change, pointing to the beneficial effects of an agreement on "the economy of the world," as well as "the quality of life of millions of people."

As more world leaders arrive in Copenhagen, security inside and outside the Bella Center, the main venue, has been intensified.

Police vans ring the convention hall, with security personnel keeping a close watch. Checkpoints have also been set up at each entrance.