Several hundred antiwar demonstrators gathered at downtown D.C. parks this morning to protest the Iraq war as it enters its sixth year, then dispersed for a daylong series of activities targeting institutions that organizers blame for prolonging the conflict.

About 9 a.m., one black-clad group of eight sat down in the intersection at 17th and L streets NW, roping themselves together with rubber tubing and blocking traffic. Police were using boltcutters and other tools to separate them from each other and make arrests. The intersection was closed to traffic until shortly after 10 a.m. At that point, the protesters began marching east on L Street.

After congregating at McPherson Square and Franklin Square, the main groups of protesters moved to the Internal Revenue Service building, the headquarters of the American Petroleum Institute, and other locations they had selected. About 20 protesters were arrested when they tried to break through security barriers and block the entrance to the IRS office at 12th Street and Constitution Avenue NW.

At the IRS, some demonstrators wore white ponchos decorated with anti-IRS slogans. Members of the group said they planned to blockade the front of the building and prevent it from opening. They blame the agency for helping fund the war.

"The war continues. People continue to die. We don't want our tax dollars spent on funding the war in Iraq and Afghanistan. I refuse to pay my taxes for it," said Ed Hedemann, 63, from Brooklyn, N.Y. Hedemann said the group hoped to shut down the IRS for the day and give employees "a vacation." He said he also hoped to be arrested.

The rally at the IRS, however, was short-lived. By 8:45 a.m., the group headed north on 12th Street NW. One of the members said they were due to participate in a "March of the Dead" at Arlington National Cemetery.

A small group of war veterans had also gathered on the Mall, while other protesters marched from Franklin Square to the offices of the American Petroleum Institute at 12th and L streets NW. The hundred or so people milled around in front of the building, sometimes impeding rush hour traffic but not trying to fully block the road. Security barriers and a phalanx of security personnel kept the entrance to the building clear, and protesters were not trying to shut down access to it.

Two days of planned antiwar protests kicked off yesterday when scores of demonstrators conducted a raucous morning march along Constitution Avenue and staged a piece of silent street theater during the evening rush hour inside Union Station.

A full day of rallies, marches, blockades and demonstrations is planned today for downtown Washington to mark the fifth anniversary of the start of the war in Iraq.

Along with blocking the IRS, activists plan to gather in front of various corporations in the vicinity of K Street between 13th and 18th streets NW. Antiwar veterans planned a 9 a.m. march on the Mall from the National Museum of the American Indian to the Capitol.

Other events are planned at the Department of Veterans Affairs, Lafayette Square and the Democratic National Committee. A "March of the Dead" from Arlington Cemetery into the District is set for 9:30 a.m. Other demonstrators will target The Washington Post and other news outlets.

Permits filed with the National Park Service by United for Peace and Justice, the umbrella group organizing the protests, estimate that crowds at the midweek demonstration will likely be in the hundreds rather than the thousands.

Yesterday's events began with a march by about 60 members of the antiwar group Code Pink, who held aloft a living room-size copy of the preamble to the Constitution, beat drums and carried peace signs. They marched along Constitution Avenue from the National Archives to the Justice Department and to the IRS, occasionally disrupting traffic.

About 30 police officers on bicycles and motorcycles and in cars followed them. No arrests were made.

"You're blocking traffic!" a police officer yelled at the protesters as they veered off the sidewalk and into the street.

"We're well aware of that!" one protester yelled back.

At 5:25 p.m., at least 100 people froze in place at the bustling Union Station: A couple kissing, a woman bending over to tie her shoe, several people pointing to maps, a couple sitting at the bar with drinks held halfway to their mouths. All stopped in mid-motion and did not move for 10 minutes in a show of support for the antiwar movement.

Passersby towing suitcases weaved their way through the tableau, dubbed "Frozen Union Station." Dozens of police officers stood by; again, no arrests were made.

"This is the strangest thing," said Michele McKnelly, a librarian from River Falls, Wis., initially unsure why everyone around her had suddenly stopped. "Then I looked around and saw some of them wearing 'End the War' T-shirts."

At 5:35 p.m., the protesters started moving and chanted "End the war" for 10 minutes, then left.

"It was a very, very positive event," said protester Deanna Gorzynski, 52, of New Milford, N.J., who is with a group called The World Can't Wait. "Even those people who don't agree with us stopped to think."