Extinction Rebellion: green protests snarl London traffic Environmental protesters have briefly blocked Waterloo Bridge in London and cut off access to other landmarks as they try to bring London to a standstill

LONDON -- Environmental protesters blocked access to several major London landmarks and vandalized oil company Shell's headquarters Monday as they tried to bring the city to a standstill.

The group Extinction Rebellion was seeking to paralyze parts of central London to highlight demands for the British government to declare a climate emergency.

Thousands of protesters converged on Marble Arch, Oxford Circus, Waterloo Bridge, Piccadilly Circus and Parliament Square. Some activists glued themselves to windows at Shell headquarters near Waterloo, and others smashed glass revolving doors and sprayed the building with graffiti.

Many carried signs pleading with the government to make fighting climate change a top priority.

Police said three men were arrested in the ongoing protests. They have not been identified or charged.

Extinction Rebellion drew attention to its cause two weeks ago when members stripped to their underwear in the public gallery in Parliament during a Brexit debate.

Protester Olivia Evershed, 23, said climate policies must be altered quickly to prevent catastrophic changes.

"If we don't do anything to change this, our children will die," she said.

Police advised Londoners to leave extra time for their journeys because of possible delays caused by the protest. Protesters aim to keep the demonstrations going for two weeks.