SPRINGFIELD, MASS., JUNE 2 -- A bitterly divided Massachusetts Democratic convention tonight allowed John H. Silber, a backer of Republicans Ronald Reagan and George Bush, to challenge two veteran party office-holders for the gubernatorial nomination in a September primary.

Silber, on leave from the presidency of Boston University, barely cleared the requirement of 15 percent of the delegate votes, to face Lt. Gov. Evelyn Murphy and former attorney general Francis X. Bellotti.

Silber qualified on the first ballot, then released his delegates, and on the second ballot Bellotti beat Murphy by a 5-4 ratio for the convention's endorsement. The result was a heavy blow to Murphy, who had predicted victory for herself.

Public polls have shown the three in a close battle, but Silber, whose political history and campaign comments antagonized many of the liberal constituencies represented in the convention, faced the threat of being barred from the ballot.

He received just 23 more votes than he needed to qualify. A fourth candidate, state Rep. John H. Flood, fell far short of the number needed to gain a spot on the ballot.

In his first race for public office, Silber has antagonized minority groups, the handicapped and welfare recipients by suggesting that permissive state policies had made them burdens on working people. He portrayed himself as an outsider challenging the liberal Democratic establishment but received crucial support from such Beacon Hill power brokers as Senate President William Bulger of South Boston.

The 15 percent requirement had been condemned by many prominent Massachusetts politicians and most of the state's newspapers. Some Democrats supporting other candidates said that if Silber were barred from the ballot, they feared a backlash that could aid Republicans in November. But there were no indications that the Murphy or Bellotti camps threw votes to Silber.

The outcome of the convention vindicated Bellotti's decision to derail a proposal from Rep. Chester G. Atkins (D-Mass.), the state party chairman, that the rules be changed to certify that all four candidates be qualified for the Sept. 18 primary ballot. The winner will face the survivor of the GOP primary between state Rep. Stephen D. Pierce, the endorsed candidate, and former U.S. attorney William F. Weld.

The start of the convention was delayed four hours when a police union that backed President Bush in 1988 threw up a picket line around the Civic Center to dramatize its contract dispute with Springfield Mayor Mary Hurley.

Gov. Michael S. Dukakis blamed the incident on the Republicans, calling it "a pretty good example" of "their strategy and frame of mind -- going negative."

The 1988 Democratic presidential nominee, whose popularity has sunk as a slowing state economy has forced ever more drastic spending cuts and tax hikes, thanked the party activists whose past support "gave me the opportunity to serve longer than any other of the governors of this commonwealth. Many of you got involved in politics because of me," he said. "I hope you are proud of that, as I am proud of you."

Dukakis was warmly cheered, but the rivals for the succession kept their distance from him. Silber criticized "the disarray of state government . . . the waste and confusion that is all too common." Bellotti recalled his 1986 warning against "managers and technocrats who . . . lose sight of our people and our soul." And Murphy, who is most closely tied to Dukakis, did not mention his name at all.

For all of Saturday morning, the police pickets held the convention hostage. State party officials, embarrassed by the dispute, blamed Ron Kaufman, who managed Bush's presidential campaign in Massachusetts and was at the convention site last night and this morning with Massachusetts Republican Executive Director Alexander Tennant. But the Democratic officials admitted they had only circumstantial evidence to support the charge. Kaufman, in jogging clothes, remarked, "I'm totally innocent, but it sure was fun."

Local 364 of the International Brotherhood of Police Officers, which drew national publicity in 1988 by joining its Boston counterpart in endorsing Bush over Dukakis, has been without a contract for a year and has had no pay raise in two years. It had informational pickets at the convention's opening session Friday night but turned it into a full-scale picket line this morning. The union said Hurley had violated an understanding by going into the convention session, something she denied.

After two hours of unavailing conciliation efforts, the Democratic state committee obtained a temporary injunction removing the pickets. But by that time, frustrated delegates were bickering with the striking policemen, occasionally pushing and shoving. At one point, the delegates jeered in unison: "Where's Bush now?"

State Rep. Stanley Rosenberg of Amherst observed, "This is another example of the unraveling of the social contract in Massachusetts. We've supported labor all these years, and now they rub our nose in it. The Republicans will use this to show the Democrats can't manage our own affairs."

Special correspondent Christopher B. Daly contributed to this report.