Less than two weeks after the indictments of the San Francisco police chief, his top three commanders and six other officers on felony obstruction of justice charges rocked this city, the district attorney today dropped the charges against the chief and his top aide, saying he could not prove they helped cover up a police probe of a street brawl allegedly involving off-duty officers.

District Attorney Terence Hallinan told a jammed courtroom that after reviewing 1,300 pages of evidence, he was dropping the charges against Police Chief Earl Sanders and Assistant Chief Alex Fagan Sr. "in the interest of justice."

Hallinan said that while the grand jury was right in implicating Sanders and Fagan in the high-level decision to remove a determined police investigator from the department's internal probe of the brawl, he could not show that the two were guilty of conspiracy to obstruct justice "with abiding certainty."

Hallinan appeared to be trying to explain why he spent several days deciding whether to pursue the obstruction case, after the San Francisco Chronicle reviewed leaked grand jury proceedings and reported that Hallinan had told the grand jury he did not have a case on the conspiracy charges.

After the hearing, Hallinan told reporters he was compelled both to file the charges, and then, on reflection, to drop them. "I did what I felt I was obligated by both law and ethics to do," he said.

Hallinan has been under intense fire from Mayor Willie L. Brown, the Police Officers Association and other critics since he began investigating the Nov. 20 fight between three young officers -- including the assistant chief's son, Alex Fagan Jr. -- and two civilians. But Hallinan asserted that he will continue to pursue charges against the other eight officers who were indicted, including five commanders and the three officers involved in the brawl outside a bar.

All pleaded not guilty last week. Sanders, who went on paid medical leave after his indictment, had suspended the other indicted officers without pay. The officers under indictment for conspiracy are deputy chiefs David Robinson and Greg Suhr, Capt. Greg Corrales, Sgt. John Syme and Officers Matthew Tonsing, David Lee and Fagan Jr.

Sanders, 65, is expected to retire soon and may not return, while Fagan, whose rookie son was allegedly at the center of the fisticuffs with two young men who said they were attacked by the officers over a bag of steak fajitas, wants to return to work, his attorney said after today's hearing.

"We're going to follow this through and ride the dragon to victory," said Bill Fazio, an attorney for Corrales, who allegedly lied about whether the three officers had been drinking that night. Fazio is also -- for the third time -- running against Hallinan for the job of district attorney.

Today's events were the latest dramatic twist in a tumultuous two weeks for the police, the district attorney and the city of San Francisco.

When the indictments were announced Feb. 28, people began scurrying around the Hall of Justice -- home of both police headquarters and the district attorney's office -- as if an earthquake had hit. In many ways, one had.

The indictments sent the police department into chaos, catapulted a back-room political grudge match between the mayor and the district attorney onto the front pages of newspapers, and divided the city into stark political camps.

Brown had been so low-key in recent months that pundits had deemed him politically irrelevant, but he came out swinging. He accused Hallinan, who had prosecuted a series of City Hall corruption cases, of political grandstanding. Others accused Hallinan of racism -- he is white and Sanders and Robinson are black.

Public opinion had appeared to be on Hallinan's side. But now, Hallinan seems to have lost the political edge. This afternoon, KGO-AM, a local news station, conducted an unscientific poll of listeners on whether the prosecutor had rightfully dropped charges against the police chief and his top deputy. Fifty-eight percent said he had not, and 38 percent said he had.

San Francisco Police Chief Earl Sanders took paid medical leave after the district attorney filed charges two weeks ago.