Extinction Rebellion activists have marched 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.
Makeshift camps at Oxford Circus and Waterloo Bridge have now been cleared. So far 69 people have been charged in connection with the protest.
Protesters left Marble Arch, which had been occupied since 15 April, carrying flags and banners and marching to drums shortly after 10:00 BST.
Police have increased restrictions at the Marble Arch site, imposing a Section 14 order - preventing protesters congregating on the road.
Campaigners said police had taken a "noticeably more confrontational attitude".
Demonstrators have previously suggested temporarily ending disruptive tactics to focus on political negotiations, as the campaign entered a second week.
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.
Ms Thunberg told a packed room her future and those of her fellow children had been "sold so that a small number of people can make unimaginable amounts of money".
The 16-year-old Nobel Peace Prize nominee added: "You don't listen to the science because you are only interested in the answers that will allow you to carry on as if nothing has happened."
The #ExtinctionRebellion procession has just stepped off from Marble Arch down Park Lane. Their final destination is Parliament Square.— MPS Events (@MetPoliceEvents) April 23, 2019
We expect some minor traffic disruption a long the route of the procession but this is being carefully managed and monitored by police
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".
The oldest person to be charged over the protests is 74 years old, while the youngest is 19. The vast majority of those charged will appear in court between 14 May and 31 May.
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 co-ordinated "die in".
Most of the demonstrators finished their lie-down protest after about half an hour.