CAMBRIDGE -- Antiwar protests and angry confrontations with police led to seven arrests on Cambridge Common yesterday as the city attempted to celebrate the 230th birthday of the US Army.

What started as a peaceful march by more than 100 sign-waving antiwar demonstrators quickly heated up after a group of protesters refused to move away from the stage and onto a grassy area designated by police. After several warnings, police lined up shoulder to shoulder, shoving some members of the group back behind jersey barriers and arresting others who refused to budge.

''They didn't have to push me," said 70-year-old Judy Summersby. ''They kept saying 'We have our orders,' and one woman out there called me trash."

Cambridge Police Commissioner Ronnie Watson defended the police action, saying that the demonstrators were physically in the way of the ceremony.

The protests outraged some spectators who brought children to the event.

''They are just ignorant, and they don't know what they are doing," said Brigit Smith, whose husband, Sergeant First Class Paul Smith of Tampa, was killed in Iraq in April 2003. He was the first soldier awarded the Medal of Honor in the Iraq war for his courage in the Battle of Bagdad Airport. The couple's 11-year-old son, David, led the Pledge of Allegiance yesterday afternoon to a backdrop of boos and sneers.

''They really made me upset," said Brigit Smith, after the ceremony. At one point during the celebration and protest, Smith crouched down to yell at a woman resisting arrest.

''I just got so mad, I couldn't help it," she said. ''My husband loved his comrades, and he loved the Army."

Cambridge has been a city known for its fierce liberalism and antiwar protests. But Cambridge Common was the site where 230 years ago many servicemen came together when the Continental Congress formally established the Army, organizers pointed out. On July 3, 1775, General George Washington arrived on Cambridge Common and took command of the newly formed Continental Army.

Yesterday, the grassy lawn was filled with Army exhibits, including live reenactments of different periods and uniforms in Army history.

Children from the Peabody Elementary School and elsewhere happily climbed in and out of camouflaged military vehicles, donning Army helmets, and acting as though they were armed soldiers. Some shook hands with service members stationed at the exhibits, and others watched in awe as parachutists leaped out of UH-60 Blackhawk helicopters overhead.

Still, the birthday celebration was overshadowed by mostly middle-age protesters chanting, ''Shame," and sporadic arrests. Meanwhile, a few 20-somethings marched around the common dressed as dead soldiers and Iraqi civilians with party hats, bullet wounds, and fake blood running down their faces.

Aligned with the demonstrators was Councilor Marjorie Decker, who called the event a veiled attempt at captivating youngsters as the Army copes with recruitment woes.

Decker, who also serves as the city's vice mayor, said she was deeply suspicious of the Army's motives, adding that members of the City Council did not know the event was happening until last week. ''There was no communication in the City Council to decide if this was an appropriate time to hold a celebration," she said.

The event was sponsored by the Massachusetts National Guard, the US Army, and the City of Cambridge.

US Undersecretary of the Army Raymond DuBois, the day's keynote speaker, said he was unmoved by the loud jeers. ''I am a Vietnam veteran, so I'm kind of used to this," he said.

''We recruit 365 days of the year," he said after his speech. ''There is only one birthday."