Chris Kitchen, a 31-year-old office worker, said he feared his treatment by police could mark the start of a clampdown on protesters, hundreds of whom are planning to travel to Copenhagen for the climate change talks in December.
Tonight he will make a second attempt to reach Denmark, where he plans to take part in discussions organised by a network of protest groups coming together under the banner Climate Justice Action.
He said he was prevented from crossing the border yesterday at about 5pm, when the coach he was travelling on stopped at the Folkestone terminal of the Channel tunnel.
Kitchen said police officers boarded the coach and, after checking all passengers' passports, took him and another climate activist to be interviewed under schedule 7 of the Terrorism Act 2000, a clause which enables border officials to stop and search individuals to determine if they are connected to terrorism.
The passports were not initially scanned, Kitchen said, suggesting the officials knew his name and had planned to remove him from the coach before they boarded. During his interview, he was asked questions about his family, work and past political activity. The police also asked him what he intended to do in Copenhagen.
When Kitchen said that anti-terrorist legislation does not apply to environmental activists, he said the officer replied that terrorism "could mean a lot of things". By the time his 30-minute interview had concluded, Kitchen's coach had gone.
Police are understood to be monitoring protesters on a number of databases, some of which highlight individuals when they pass through secure areas, such as ports.
Kitchen is a prominent activist who has taken place in a number of peaceful acts of civil disobedience, such as glueing himself to a statue in parliament, to call for more action to cut carbon emissions.
"The use of anti-terrorist legislation like this is another example of political policing, of the government harassing and intimidating people practising their hard earned democratic rights," he said. "We are going to Copenhagen to take part in Climate Justice Action because we want to protest against false solutions like carbon trading and to build a global movement for effective, socially just solutions.
"People who are practising civil disobedience on climate change in the face of ineffectual government action are certainly not terrorists, and I am sure that their actions will be vindicated by history."
Kitchen said police paid for a ticket for him to return to London after questioning and arranged for the coach company to give him a seat on another coach.
A Home Office spokesman said: "There has been no change in policy. Schedule 7 of the Terrorism Act 2000 enables an examining officer to stop, search and examine a person at a port or in a border area to determine whether they are someone who is or has been concerned in the commission, preparation or instigation of acts of terrorism.
"The exercise of the powers by the police is an operational matter for each force."
Jack Sheppard adds "On our second try to get to Copenhagen we we're reminded that we were relatively lucky, when 2 "Sans Papiers" were removed from our coach - probably to a German Detention Centre"