When Tony Blair honoured the dead at the Cenotaph in Whitehall on Remembrance Day nobody interfered. But when a peace activist tried to do the same in virtually the same place, two police sergeants and 12 constables arrived in haste to arrest her.

When a mother said on the radio that she thought gay couples should not adopt children, the police were soon in touch. When an evangelist was physically attacked for carrying a placard denouncing homosexuality, he was the one prosecuted. And when a former No 10 spin doctor revealed that the prime minister had once blurted out “f****** Welsh”, he too found the boys in blue wanting to know more.

What is going on? Has Britain, regarded around the world as the home of free speech, become a country in which people can no longer say what they think? Have ancient freedoms of movement and self-expression vanished? And why do the police have these new powers? The answers lie in the government’s slippery reaction to two distinct strains of zealotry: Islamic terrorism and political correctness.

It has exploited the mood of insecurity to push through a law protecting itself from public protest; and it has turned the police into creatures of the PC lobby by putting them in the front line against “wrong thinking”.

Three years ago Tony Blair presented himself as a friend of free speech. “I pass protesters every day at Downing Street and believe me, you name it, they protest against it,” he told an American audience. “I may not like what they call me but I thank God they can. That’s called freedom.”

It seems we have lost that freedom. The Serious Organised Crime and Police Act 2005 makes it illegal to protest within 1km of Parliament Square without police authorisation. It was ostensibly designed to evict Brian Haw, who came to Parliament Square in 2001 with a plastic chair and a sign, and has kept a vigil there ever since.

David Blunkett, proposing this law as home secretary, joked that it was “a sledgehammer to crack a nut”. The sledgehammer failed: courts ruled that the act could not be applied retrospectively. Haw’s campaign continues.

Having got the powers, however, the police are using them. Some 20 cases are coming up for trial. Maya Evans’s was the first. She was arrested for trying without permission to read the names of civilians and soldiers killed in Iraq at the gates of Downing Street in October.

One of the arresting officers told her the new act was brought in to help police organise a busy area. “He presented the scenario of a pro-war group and an anti-war group demonstrating at the same time outside Downing Street. He asked what I thought might happen if the two groups came into conflict and who would have to step in to protect my safety. So I asked if I had been arrested for my own safety. He said yes.”

She was taken to a police cell and held for nearly six hours. This month she was given a conditional discharge but ordered to pay £100 costs.

Lord Falconer, the lord chancellor, claimed last week that Evans’s arrest was “a sensible measure to avoid disorder round parliament”. He added: “Freedom of speech is alive and well in this country.”

Among the many lawyers who disagree is Lucy Moorman, who says the 1986 Public Order Act already gave the police power to intervene when protests presented real danger. By contrast, the new legislation offers parliament special protection from dissent.

Moorman has given lectures in eastern Europe challenging local laws that ban criticism of political leaders. She had “tried to explain that the president should not be entitled to special protection from criticism but should be the most tolerant . . . Well I used to preach quite smugly four years ago, but I don’t feel so smug any more”.