WASHINGTON - Convinced that this is their moment, tens of thousands marched yesterday in an antiwar demonstration linking military families, ordinary people, and an icon of the Vietnam protest movement in a spirited call to get out of Iraq.

Celebrities, a half-dozen lawmakers, and protesters from distant states rallied in the capital under a sunny sky, seizing an opportunity to press their cause with a Congress restive on the war and a country that has turned against the conflict.

Marching with them was Jane Fonda, in what she said was her first antiwar demonstration in 34 years.

"Silence is no longer an option," Fonda said to cheers from the stage on the National Mall. The actress, once derided as "Hanoi Jane" by conservatives for her stance on Vietnam, said she had held back from activism so as not to be a distraction for the Iraq antiwar movement, but needed to speak out now.

There also were antiwar rallies in dozens of cities across the country, including Los Angeles, Chicago, Boston, Denver, Miami and New Orleans.

The rally on the Mall unfolded peacefully, although about 300 protesters tried to rush the Capitol, running up the grassy lawn to the front of the building. Police on motorcycles tried to stop them, scuffling with some and barricading entrances.

Protesters chanted "Our Congress" as their numbers grew and police faced off against them. The demonstrators later joined the masses marching from the Mall, around Capitol Hill and back.

United for Peace and Justice, a coalition sponsoring the protest, had hoped 100,000 would come. Police, who no longer give official estimates, said privately that the crowd was smaller than that.

At the rally, 12-year-old Moriah Arnold stood on her toes to reach the microphone and tell the crowd: "Now we know our leaders either lied to us or hid the truth. Because of our actions, the rest of the world sees us as a bully and a liar."

The sixth grader from Harvard, Mass., organized a petition drive at her school against the war that has killed more than 3,000 U.S. service members, including seven whose deaths were reported yesterday.

More Hollywood celebrities showed up at the demonstration than buttoned-down Washington typically sees in a month.

Actor Sean Penn said lawmakers would pay a price in the 2008 elections if they did not take firmer action than to pass a nonbinding resolution against the war, the course Congress is now taking.

"If they don't stand up and make a resolution as binding as the death toll, we're not going to be behind those politicians," he said. Actors Susan Sarandon and Tim Robbins also spoke.

Fonda was a lightning rod in the Vietnam era for her outspoken opposition to that war and her advocacy from Hanoi during the conflict. Sensitive to the old wounds, she made it a point to thank active-duty service members, veterans, and Gold Star mothers who attended the rally.

She drew parallels to the Vietnam War, citing "blindness to realities on the ground, hubris . . . thoughtlessness in our approach to rebuilding a country we've destroyed." But she noted that this time, veterans, soldiers and their families increasingly and vocally were against the Iraq war.

The House Judiciary Committee chairman, Rep. John Conyers (D., Mich.), threatened to use congressional spending power to try to stop the war.

"George Bush has a habit of firing military leaders who tell him the Iraq war is failing," he said, looking out at the masses. "He can't fire you."

Referring to Congress, Conyers added: "He can't fire us. The founders of our country gave our Congress the power of the purse because they envisioned a scenario exactly like we find ourselves in today. Not only is it in our power, it is our obligation to stop Bush."

On the stage rested a coffin covered with a U.S. flag and a pair of military boots, symbolizing American war dead. On the Mall stood a large bin filled with tags bearing the names of Iraqis who have died.

A small contingent of active-duty service members attended the rally, wearing civilian clothes because military rules forbid them from protesting in uniform.

Air Force Staff Sgt. Tassi McKee, 26, an intelligence specialist at Fort Meade, Md., said she had joined the Air Force because of patriotism, travel and money for college.

"After we went to Iraq, I began to see through the lies," she said.

In the crowd, signs recalled the November elections that defeated the Republican congressional majority in part because of President Bush's Iraq policy.

"I voted for peace," one said.

"I've just gotten tired of seeing widows, tired of seeing dead Marines," said Vincent DiMezza, 32, wearing a dress Marine uniform from his years as a sergeant. A Marine aircraft mechanic from 1997 to 2002, he did not serve in Iraq or Afghanistan.

About 40 people staged a counterprotest, including Army Cpl. Joshua Sparling, 25, who lost a leg to a bomb in Iraq.

He said the antiwar protesters, especially those who were veterans or who were on active duty, "need to remember the sacrifice we have made and what our fallen comrades would say if they are alive."

Protest organizers said the crowd included people who came on 300 buses from 40 states.