Mark Barrett said he would not pay the fine

Mark Barrett was arrested in August 2005 for breaking the new law, which states protests within a half-mile of Parliament need permission.

He was convicted at Bow Street Magistrates Court of unlawful protest, but said he did not intend to pay.

"It's made me more determined to campaign and win," he told the BBC.

Since the Serious Organised Crime and Police Act came into effect in August several protesters have been arrested and charged.

The protest tea parties take place every Sunday

Unwary cabbies who had intended to drive around Parliament Square in protest at proposed law changes called it off after a warning from police.

Mr Barrett has been organising weekly protest tea parties in Parliament Square and said he was arrested "while sitting on the grass drinking tea and eating cake".

The government say the law is aimed at tightening up security around the Houses of Parliament, but its critics say it infringes the right to freedom of speech.

Anyone wanting to demonstrate in a half-mile zone around the square must have prior permission from the police.

Anti-war campaigners have set out to test the law, reading out the names of children orphaned or killed in Iraq, or holding protest carol services.