Police were forced to lock the doors to Parliament after protesters tried to force their way inside.

Scotland Yard said none of the environmental demonstrators, who are campaigning against airport expansion, managed to breach security at the Palace of Westminster.

Doors at the main St Stephen's entrance were closed and secured with two large metal bars, with the hammering of protesters clearly audible from the inside.

The protest was organised by the Climate Rush group, which is angry about airport expansion and other Government policies on the environment.

Climate Rush organiser Jennifer Ruhemann said the action was by a broad coalition of groups including the Women's Institute, the Green Party and Plane Stupid.

Many of the protesters were dressed as Emmeline Pankhurst, leader of the suffragette movement, and said they wanted to "rush" Parliament to mark the centenary of similar action by the suffragettes.

The latest attempt to breach security comes months after anti-aviation group Plane Stupid gained access to the roof of Parliament, unfurling banners and chaining themselves to the building.

A Met Police spokesman said six people were arrested as a result of the protest.

He added: "Police were aware that this group intended to lobby their MPs, as is their democratic right.

"On arriving at the St Stephen's Gate entrance the crowd attempted to rush the entrance.

"A pre-deployed line of police officers prevented this from happening. Additional officers were put in place to start to disperse the crowd from the Palace of Westminster."