Protesters in Washington on Sunday urged President Barack Obama to reject the Keystone XL pipeline that would carry oil from Canada to Texas.
They also wanted him to order the Environmental Protection Agency to set carbon standards for power plants.
The event was organised by environmental groups under an umbrella group named Forward on Climate.
The Sierra Club said 30,000 demonstrators were at the event. Participants arrived aboard buses from 28 states.
Canadian actress Evangeline Lilly (Lost) called on Mr Obama to stop being a puppet of the oil companies.
