Copenhagen shopowners fear a large demonstration on the sidelines of UN climate talks on Saturday will turn violent and result in vandalism, a police official said.

"Those who in the past have seen their windows smashed are afraid that troublemakers will descend on the streets, and we can only advise them to protect their storefronts if their shops are on the demonstration route," police spokesman Lars-Christian Borg told AFP.

US-owned stores such as 7-Eleven and McDonald's as well as banks have in the past proven to be vandals' favourites during demonstrations. The shops most at risk have already taken steps to protect their storefronts or have hired guards to patrol the area outside.

Organisers have said they hope the December 12 demonstration -- which is expected to gather between 40,000 and 80,000 people depending on various estimates from police and organisers -- will be "peaceful."

But violent far-left groups have already threatened via internet sites to swarm the city to protest against the climate negotiations, which they say do not address the concerns of the poor countries most threatened by global warming.

Police have warned potential agitators that they will respond "firmly" to acts of violence.

"We will not allow the troublemakers to rule," Borg said, recalling that police have extended detention times to 12 hours and increased fines and prison sentences for disturbing the peace.

Denmark has also reinstated land and sea border controls at its borders with neighbouring Germany, Sweden and Norway in a bid "to prevent potential troublemakers from entering the country."

The controls will remain in place "as long as we deem necessary," Borg said.