Extinction Rebellion activists are marching on Parliament to invite MPs to discuss climate change policies.
More than 1,000 people have been arrested since the protests began in central London a week ago.
Police said a “robust” plan was in place and protesters must leave Parliament Square by midnight.
Protesters left Marble Arch, which had been occupied since 15 April, carrying flags and banners and marching to drums shortly after 10:00 BST.
Demonstrators have previously suggested temporarily ending disruptive tactics to focus on political negotiations, as the campaign enters a second week.
Makeshift camps at Oxford Circus and Waterloo Bridge have been cleared, but activists remain at Marble Arch.
Jay Monk, 49, who is taking part in the march, said it was a signal of a “renewed push” to have politicians answer the group’s demands.
Campaigners have three core demands: for the government to “tell the truth about climate change”; to reduce carbon emissions to zero by 2025; and to create a citizens’ assembly to oversee progress.
The Met Police said anyone failing to comply with its condition to leave Parliament Square by 23:59 would be “committing an offence and will be liable to arrest”.
Teenage activist Greta Thunberg has invited party leaders to a meeting in the House of Commons.
The 16-year-old Nobel Peace Prize nominee is expected to meet the leaders at around 11:30 before giving a speech at a meeting in Portcullis House at about 14:00.
MPs are returning to Parliament after a week-long recess.
Protester Steve Jones, 53, suggested some of the disruption might be relaxed if the group was granted “meaningful discussions” with the government.
On Monday, at least 100 protesters laid down under the blue whale skeleton at the Natural History Museum in a coordinated “die in”.
Most of the demonstrators finished their lie-down protest after about half an hour.