LONDON – Pro-hunting demonstrators descended on Westminster yesterday, storming into the House of Commons chamber and causing chaotic scenes inside and outside Parliament in a foretaste of a countryside uprising against a fox-hunting ban.

After the worst breach of Commons security in living memory, armed police were placed at all entrances to the chamber – the first time lawmakers have had such a guard.

Five demonstrators evaded recently upgraded security to reach the floor of the chamber, using a forged letter that purported to show they had been invited to a site meeting to discuss building works inside the Palace of Westminster.

In all, it is believed nine men entered the building wearing hard hats and suits, but four were stopped from entering the chamber.

Friends of the men, who included Luke Tomlinson, a friend of Prince William and Prince Harry and captain of the England polo team, and Otis Ferry, the son of the rock singer Bryan Ferry, said they had given themselves only a 10% chance of success.

The supporters of the invaders said they were astonished that on a day when there was a full-scale political protest outside Parliament, security could be so lax within.

The men reached the speaker’s chair and the front benches, only inches from where Prime Minister Blair, the Cabinet, and opposition leaders had been sitting four hours earlier.

Shocked lawmakers and ministers watched as one protester harangued the rural affairs minister who is promoting the ban, Alun Michael.

Commons officials rushed forward to tackle the T-shirted protesters.

Violence had broken out minutes earlier in Parliament Square. Nineteen people were injured, two of them police officers, and 11 demonstartors were arrested for public order offenses.

The security breakdown and violence occurred only three days after a demonstrator dressed as Batman breached the security cordon around Buckingham Palace.

But the lapse in Commons security was far more serious than Monday’s incident at the palace, the throwing of flour bombs at Mr. Blair from the visitors’ gallery in May, or the scaling of Big Ben in an anti-war protest in March.

Had the hunt protesters been armed intruders, they could have killed or injured lawmakers and ministers – having gained access to the most secure areas of the Commons.

Lawmakers said it was the first violent intrusion on the floor of the Commons since the reign of Charles I – when the King entered the chamber on January 4, 1642, in an unsuccessful attempt to arrest five members.

Although the demonstration started in good-natured fashion, the police were caught off guard by the scale of the protest. The organizers claimed there were 20,000 protesters but the police put the figure at half that. Inside the Commons, the attendance was thin as lawmakers rushed the bill to ban hunting.

The Hunting Bill was given a second reading by 356 votes to 166, a majority of 190. Mr. Blair remained in Downing Street and did not venture the few hundred yards through the angry crowd to the Commons to vote.

The scale and intensity of the protests – which drew comparisons with the poll tax demonstrations when Prime Minister Thatcher was in power – surprised and alarmed lawmakers.

It was seen as a sign that many members of a normally law-abiding “middle Britain”‘ might be prepared for a campaign of civil disobedience if the ban goes ahead.