WASHINGTON (AP) _ Eleven defendants - the largest number in a Washington murder case - go on trial this week on charges of beating a 49-year-old woman to death in a robbery a few blocks from the U.S. Capitol.

''There has never been a case of this enormity, with this many co- defendants in the city,'' said Alan Palmer, a defense lawyer. ''In New York and down here, they routinely try 10 to 20 people in narcotics cases, but never murder.''

The 11 are accused of the Oct. 1, 1984 murder of Catherine Fuller, a mother of six. Police said she was attacked on a street, dragged into an alley and beaten and kicked as her clothes were ripped from her body and a pipe rammed into her rectum.

The D.C. Medical Examiner said the woman died of ''blunt force injuries,'' including a torn liver, punctured ribs, multiple bruises and a torn rectum wall.

Police investigators said the defendants were members of a loosely organized gang called the ''Eighth and H Crew'' after a street corner in their neighborhood 10 blocks from the Capitol.

When 100 potential jurors were given a brief description of the crime 20 indicated they could not be fair and impartial because of the nature of the case.

Two men have pleaded guilty to second-degree murder charges, but a 17-year- old female and 10 males - many of them teenagers - are set to stand trial in District of Columbia Superior Court as adults on charges of first-degree murder, kidnapping and armed robbery.

''This is unprecedented,'' said Steven Kiersch, one of a dozen defense attorneys. ''No one has ever handled anything like this. It's the largest indictment on a single homicide I've ever heard of.''

The trial, beginning early this week after jury selection, is expected to take about six weeks.

Minutes before jury selection began on Friday, Calvin L. Alston, 20, pleaded guilty to second-degree murder, admitting he served as a lookout and kicked the woman a few times as she was dragged into the alley. Alston agreed to testify against the other defendants.

Assistant U.S. Attorney Jerry Goren told the court that the group was standing in a park when they saw Mrs. Fuller put money in her bra. They followed the victim, and two of the defendants grabbed her and pushed her into the alley.

Other members of the group came in from both sides of the alley and took part in the attack, Goren said.

Several defense lawyers said the Alston plea could have a ''domino effect,'' sending others to the prosecutors with deals in hand. But each said their client would go ahead with the trial.

During pre-trial motions, some of the defendants giggled and chatted; some quietly read bibles during breaks in the proceedings.

Federal marshals with metal detectors have been screening spectators and security is expected to get even tighter when the trial begins and relatives of the defendants come to watch.

First-degree murder convictions are punishable by 20 years to life in prison; armed robbery by 15 years to life and kidnapping carries a maximum sentence of life in prison.