Police have been forced to close the main public entrance to Parliament after environmental protesters attempted to force their way in.
Up to 600 demonstrators, including some against the expansion of Stansted and Heathrow airports, took part.
Officers made six arrests and have shut the St Stephen's Gate entrance.
Organised by the Climate Rush group, the aim was to emulate the attempt by the Suffragette movement to storm Parliament 100 years ago.
At about 1830 BST, demonstrators marched to Parliament where they were stopped by police at the St Stephen's entrance.
"We simply wanted to exercise our legal right to make our representations to Parliament"
Police said none of the environmental demonstrators had managed to breach security.
Inspector Tim Barfoot, who was on the scene, said: "The situation is being dealt with.
"We are concerned to ensure the effective running of Parliament."
Five people have been arrested for breach of the peace and a 23-year-old woman has been held for breach of bail, a Metropolitan Police spokesman said.
Doors at the main St Stephen's Gate entrance were closed and secured with two large metal bars. Protesters striking the doors were clearly audible from the inside.
About 400 demonstrators from a number of organisations had gathered during the afternoon for a protest and rally.
'Peaceful protest'
One protester, Rosie Haworth-Booth, 62, from Wandsworth, south-west London, said: "It's a pity we couldn't get into Parliament. I was near the front and got a bit afraid. I'm too old for this.
"The government is prepared to give £50bn to help the banks and more people will be affected by this issue."
Carole Barbone, of Stop Stansted Expansion, said: "I can understand the police might have felt there was a risk due to the numbers of people involved but there was no intention from anyone involved to cause any damage or harm to people or property.
"We simply wanted to exercise our legal right to make our representations to Parliament."
The government gave the go-ahead to Stansted's expansion this month and a decision on Heathrow is due this year.
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