Singing hymns, clapping hands and chanting for their cause, hundreds of antiabortion demonstrators delayed the opening yesterday of two District medical clinics where abortions are performed, and 343 protesters were arrested.

The demonstrators, who call their action Operation Rescue, were resisted by members of the National Organization for Women, the Washington Area Clinic Defense Task Force and others, who ringed clinics and stood at doors to keep the buildings open.

Traffic on the western side of the District was snarled during the morning rush hour, but no violence was reported during the protest. There was at least one injury: Protester Patricia Weaver, of St. Louis, suffered a bruised shoulder while she was being arrested and was treated at George Washington University Medical Center, a hospital spokeswoman said.

Both sides on the abortion issue have refined their protest and counter-protest tactics over the last several years as the strategy of blockading clinics emerged across the country.

Yesterday's activities followed a pattern similar to the protests that occurred almost exactly one year ago in the District.

At that time, a larger group of Operation Rescue activists were able to close one clinic completely and delay the opening of two others.

More than 700 demonstrators were arrested, and morning commuter traffic was tied up for hours.

Yesterday, about 500 protesters were confronted by several hundred counter-demonstrators at two clinics. Both remained open.

"They claim the clinic was open and they probably did do that," Operation Rescue spokeswoman Chris Santo said, referring to those who defended access to the Washington Surgi-Clinic near Washington Circle. "I am sure the killing went on inside."

Santo said there would "most definitely" be other clinic blockade attempts today, but as is usually the case, the locations were not revealed in advance.

The clinic blockades often involve courtroom action as well as street demonstrations.

Larry Eisenstein, a lawyer representing NOW and Planned Parenthood, said yesterday he thought the Operation Rescue demonstrators violated an injunction against blockades issued in May by U.S. District Judge Louis F. Oberdorfer.

"I fully anticipate there will be contempt proceedings," Eisenstein said.

Federal judges in the District, Virginia and Maryland have issued permanent injunctions against Operation Rescue, its founder, Randall Terry, and several of its other leaders to prevent them from blocking entrances or exits to clinics and organizing protests to blockade clinics.

Yesterday, abortion rights advocates arrived about 5 a.m. at the Planned Parenthood clinic at 1701 Q St. NW near Dupont Circle, an hour ahead of the antiabortion forces. About 70 men and women linked arms and stood across the entrance of the clinic. Across the street, the antiabortion activists stood five deep singing "The Battle Hymn of the Republic" and waving signs that said "Jesus says let the children come unto me" and "Stop the killing."

Standing in line guarding the clinic was Shelly Wilse, an organizer for the Washington Area Clinic Defense Task Force, who said she is a veteran of such face-offs.

"We solely do defense," she said of her group. "We escort women every week into clinics. These people show up every Saturday and hold prayer vigils at the clinics."

Police arrested one person at Planned Parenthood on a charge of crossing a police line.

Activists on both sides of the issue moved on to the second clinic about 7:30 a.m.

At the Washington Surgi-Clinic building, said Hannah Olanoff, president of the D.C. state office of NOW, the abortion rights advocates held one of the two doors while antiabortion protesters held the other. She said her side left their doorway only when threatened with arrest by D.C. police. She said police were unfair in allowing protesters to take their place at the doorway.

From inside the clinic, five women peered out of second-floor windows holding signs that read "Pro-Choice."

Terry arrived at the Washington Surgi-Clinic after hundreds of those in his group were already in place blocking two entrances to the clinic. He came to a stop at the police line about 20 yards from the clinic's entrance.

Asked whether he was concerned about the injunction, Terry said: "We have our own injunction. God's injunction."

Olanoff declared victory after the first patient entered the Washington Surgi-Clinic at 10 a.m., two hours after the usual opening time.

"It is a victory for us because women got into the clinic," she said.

Laurie Welling, clinic director, said three of the eight patients expected yesterday made it through the demonstrators. She said the others had called to reschedule their appointments.

The demonstrators arrested outside the Washington Surgi-Clinic were taken to D.C. Superior Court, where they were processed on the misdemeanor charges and fined $50. The fines must be paid within 15 days.

Most of them were released. A police spokesman said demonstrators who had outstanding warrants for other offenses or from last year's protest would not be released immediately.