Fifteen environmental protesters, who blocked the roads outside the Houses of Parliament in the first stage of an “escalating campaign of civil disobedience” have been arrested.

More than 1,000 people from the newly formed Extinction Rebellion group issued a declaration of rebellion in London’s Parliament Square on Wednesday in an effort to draw attention to the unfolding climate emergency.

The group, that included families, pensioners and teenagers, then sat down in the road blocking traffic at one of London’s busiest intersections for more than two hours as speakers from different faiths, Green party politicians and journalists warned about the unravelling environmental crisis.

Green MEP Molly Scott Cato told the crowd: “We are here because we have a deep love and spiritual connection to the planet.

“We must rise up. When the law tells you to support something that your deep inner voice is fundamentally against, you should take non-violent direct action.”

Other speakers included Guardian columnist George Monbiot and Green party MP Caroline Lucas.

The protest came a day after a stark warning from scientists that humanity has wiped out 60% of mammals, birds, fish and reptiles since 1970 – a level of extinction that experts warn threatens the future of human civilisation.

And earlier this month the UN warned that there are only 12 years left to prevent climate catastrophe.

Wednesday’s protest was due to be a symbolic start to a campaign of civil disobedience that organisers say will bring large parts of London to a standstill next month.

The group had expected a few hundred people to turn up but more than 1,000 arrived and after a show of hands it was decided to block the road outside parliament.

Some of those involved locked themselves together, while others linked arms as the police moved in.

Felix, 28, was one of those who had chained himself to a fellow protester. He said the disruption, which saw parts of central London come to a standstill, was justified.

“The disruption we are causing today is nothing to the destruction that our governments are unleashing by not taking serious steps to stop the ecological crisis. I have never been to prison before but I feel I have to try and do something.”

The group is planning a campaign of mass civil disobedience starting next month and promises it has hundreds of people who are prepared to get arrested in an effort to draw attention to the unfolding climate emergency. It will culminate with a sit-in protest in Parliament Square on 17 November and a series of planned occupations of London bridges.

The “rebellion” has been backed by almost 100 senior academics from across the UK, including Rowan Williams, the former archbishop of Canterbury.

In a letter published in the Guardian last week they said the failure of politicians to tackle climate breakdown and the growing extinction crisis means “the ‘social contract’ has been broken … [and] it is therefore not only our right, but our moral duty to bypass the government’s inaction and flagrant dereliction of duty, and to rebel to defend life itself.”

Organisers said Wednesday’s turnout showed the strength of frustration and the desire to do something to tackle the ecological crisis.

And they predicted widespread disruption next month.

“People are no longer prepared to sit back and watch our politicians and corporations drive us off the cliff of ecological destruction,” said one of the organisers, Gail Bradbrook. “This is an emergency situation and it needs an emergency response.”