Now is the time for all good liberals to act in anger for the sake of their country, the Rev. Robert F. Drinan, president of Americans for Democratic Action, said yesterday.

"The present moment is for liberals the worst moment in 50 years," Drinan said in remarks prepared for ADA's 36th annual convention here. " . . . Liberals are angry, and they need to express and act on that anger."

The former Massachusetts Democratic congressman, who earned a reputation for fiery oratory and liberal crusades--if not for understatement--during the decade he served in the House, suggested that liberals should vent their anger not only now but on the 1,000th day of the Reagan administration.

"Oct. 17, 1983, is a target date when the liberals, the poor, the women, the blacks and all of the other millions who have been hurt by the cruelty of the Reagan administration must proclaim their indignation and their determination that they will no longer allow their own government to rob them of their rights, their dignity, their future," Drinan said.

The ADA is planning to commemorate Reagan's 1,000th day in office by issuing its assessment of the administration's accomplishments. Last night's speech served as a partisan preview.

"The destructiveness and the moral bankruptcy of the Reagan administration continues to be unbelievable and shocking," Drinan said. " . . . This administration is trying to dismantle and destroy programs which all but a few citizens in America had deemed to be permanent parts of the American consensus.

" . . . The time has now long since arrived when liberals have to scoff in indignation at the preposterous contention that the vast majority of social programs 'have not worked.' These programs have reduced poverty, decreased hunger, cut back children's diseases, extended to millions of people some type of adequate housing . . . . "

Despite the heavy overlay of anti-Reagan rhetoric, the convention's opening day was free of the sort of presidential politicking that has become a staple of the ADA convention diet. Rules were debated that would allow a presidential endorsement by the ADA this fall, at the earliest, but no candidates' names were mentioned.

And as the delegates shuttled between conference rooms at the Capital Hilton, Ann Lewis, political director of the Democratic National Committee, noted the absence of candidate representatives and campaign emblems.

"It's an ADA convention, but nobody's even wearing buttons for presidential candidates," she said. "I'm afraid that when it comes to presidential politics, the liberal wing of the Democratic Party has become somewhat like Sherlock Holmes' dog that didn't bark."