Be sure to check out our live blog of last night's 'Occupy' events here.

BOSTON (Reuters) - Tension mounted between protesters and police on Monday night after Occupy Boston members expanded their footprint in downtown Boston, then said they were being pressed by police to backtrack.

A few hundred police were on the scene, and protesters said authorities had given participants at the rally an ultimatum to return to their original encampment or be moved along.

"The BPD respects your right to protest peacefully. We ask for your ongoing cooperation," the Boston Police Department said in a tweet to @Occupy_Boston, but did not mention any ultimatum.

Protesters' tents have been set up in Dewey Square Park in downtown Boston all month, but on Monday expanded to a larger section of the nearby Rose Fitzgerald Kennedy Greenway. Many linked arms Monday evening in a show of solidarity on their expanded turf.

Boston earlier saw one of its biggest rallies so far in a movement that began in New York last month to protest against perceived Wall Street excesses and other social issues and has spread to cities across the nation. Hundreds of protesters, including many college students, marched in support of Occupy Boston.

Protests across the country have objected to what they see as an unacceptable income gap between rich and poor. They also have complained about the Wall Street bailout in 2008, which they say aided banks while average Americans suffered under high unemployment and job insecurity.

In fliers handed out on site in Boston, and in a press release, the police told protesters: "if asked to leave an area, please do so peacefully."

Police said officers would arrest those knowingly in violation of the law "if necessary." They also warned protesters they would use video cameras to record any disorderly behavior.

In return, protesters urged their followers to call police and the fire department to express support for Occupy Boston and "come down to Dewey Square right now!"

(Reporting by Ros Krasny and Adam Hunger; Editing by Cynthia Johnston)

Mochila insert follows.