WASHINGTON (Reuters) - Six people were arrested on Tuesday during a protest in a Senate office building involving antiwar and Occupy DC demonstrators, police and participants said.

The group of about 100 protesters unfurled upside-down flags -- a symbol of distress -- and antiwar banners in the atrium of the Hart Senate Office Building, which houses senators' personal offices and some committee rooms.

They chanted "We are the 99 percent" and "Stop the wars" until Capitol police cleared the area, participants and observers said.

The protest had been planned for months by such antiwar groups as Veterans for Peace and Code Pink to mark the 10th anniversary of the invasion of Afghanistan.

However, the event drew participants as well from the Occupy DC protest, a local version of the anti-corporate Occupy Wall Street demonstrations that have spread nationwide.

The "99 percent" chant has been a feature of those demonstrations, reflecting the protesters' view that the country's wealthiest one percent have too much and are too powerful.

The Washington protesters said there was a common philosophy underlying the antiwar and anti-Wall Street movements.

"We certainly want to end the wars, but our message is much broader than that. There's a corrupt corporate America that's running this country. One percent are making the decisions for the rest of us," said Mike Tork, a Vietnam veteran from Falmouth, Mass., who is involved with Veterans for Peace.

Police arrested six protesters and charged them with unlawful conduct for demonstrating inside a Capitol building, a police spokeswoman said.

Over the past weeks, demonstrators from different protests have set up camps at two sites in downtown Washington near the White House.

The number of Occupy DC protesters camped out in McPherson Square a few blocks north of the White House had risen to about 60 at midday on Tuesday, well above the dozen or so there last week.

The site also showed more permanence, with tents, tarps, sleeping bags, a food table and piles of other equipment laid out.

"I'm hard pressed to think what could be offered to appease people who are showing up here and all across the country. They aren't going to be appeased by the lesser of two evils," Peter Vos, a 52-year-old teacher of computer skills to prison inmates, said after he dropped off 100 ponchos for protesters.

At a separate protest with aims similar to that of Occupy DC, the Park Service has offered the Stop the Machine movement a four-month extension of its occupation of Freedom Plaza in central Washington, organizer Dr. Margaret Flowers said.

"We feel that we're part of the same movement. The most important thing that we're doing is getting people out and protesting," Flowers told Reuters.

A Park Service spokesman was not available to comment.

(Reporting by Andy Sullivan and Ian Simpson; Editing by Jerry Norton)