(Updates Pelosi re-elected House speaker, more details)

By Thomas Ferraro and Richard Cowan

WASHINGTON, Jan 6 (Reuters) - Democrats celebrated their increased clout in the U.S. Congress on Tuesday, swearing in new members who are expected to help President-elect Barack Obama enact a massive economic stimulus package.

Gavels sounded at about noon EST (1700 GMT), opening the Senate and House of Representatives a day after Obama visited the Capitol to press Republicans as well as fellow Democrats for quick action on a $775 billion package of tax cuts and new spending to stem a deepening recession.

Obama takes office on Jan. 20, which would give Democrats control of the White House and Congress for the first time in 14 years.

"We need action, and we need action now," Nancy Pelosi, a California Democrat, declared after colleagues elected her to a second term as House speaker.

Despite such challenges as the ailing economy, two wars and an estimated 46 million Americans without health insurance, the opening day of Congress amounted to somewhat of a victory party for the Democrats who expanded their control of the Senate and House in the November election.

"We are grateful to begin anew with a far more robust Democratic majority," said Senate Majority Leader Harry Reid.

Reid hailed Democrats' expanded majority, but echoed Obama's call for "change in Washington" after years of partisan bickering and gridlock.