A statement, Scotland Yard said: "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. None of this group gained access to the Palace of Westminster.
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.
In addition to the police presence on the streets, armed officers patrolled the hall within.
The rally, organized by the group Plane Stupid and entitled Climate Rush, was designed to coincide with the 100th anniversary of the suffragettes going to Parliament to demand change and the protesters were all women.
One of those women, Rosie Haworth-Booth, 62, from Wandsworth, south-west London, told the BBC: "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.
Protesters from many communities concerned about the British Government's disregard for climate change are demanding an end to airport expansion (the government gave the go-ahead to Stansted's airport expansion this month and a decision on Heathrow is due this year). and other measures that would frustrate national efforts to reduce global warming emissions. They came with the following three demands:
1. No airport expansion.
2. No dirty coal fired power stations.
3. Cross-party policy in line with the most recent climate research and science.
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 call for the action read:
"Normal people really can change the world. 100 years ago the Suffragettes went to Parliament and demanded that their society change. They held a mass rally outside Parliament to which thousands came, before a number of them rushed into Parliament and got everyone's attention. We invite you all to celebrate the 100th anniversary of this world changing event. We invite you to celebrate when women got radical. Enjoy a rally and speeches in Parliament Square. Enjoy social change."
The following is from the Press Association.
Protesters try to enter Parliament
Police were forced to lock the doors to Parliament after protesters attempted to force their way into the Palace of Westminster.
Scotland Yard said none of the environmental demonstrators, campaigning against airport expansion, had managed to breach security.
There was one confirmed arrest - of a 23-year-old woman for breach of bail.
A spokesman said an "appropriate policing plan" had been put in place to deal with the protest, which had been expected.
Met Police 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."
Asked whether any protesters were able to get in to the Palace, he said: "No, we were able to deal with the situation."
There had been a larger than usual police presence at Westminster throughout the afternoon in readiness for the protest.
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 latest attempt to breach security comes months after the anti-aviation group Plane Stupid gained access to the roof of Parliament, unfurling banners and chaining themselves to the historic building.