Lawyers for Rep. Daniel J. Flood (D-Pa.) opened their defense presentation yesterday by producing a surprise witness who directly contradicted some of the most crucial prosecution testimony in Flood's bribery, conspiracy and perjury trial.

Flood could not have accepted a $2,000 cash bribe on Oct. 31, 1974, at a private meetingwith a Pennsylvania developer, according to the testimony of Kevin J. Clark, because the congressman was with Clark all that day.

Clark acknowledged under sharp cross-examination, however, that his testimony was prepared only three days ago with the help of his brother, Michael Clark, who is Flood's press secretary.

Kevin Clark held nonetheless firmly to his statements, recalling precisely his entire schedule with Flood on the critical day, down to 15-minute and 20-minute segments.

Other defense witnesses yesterday in U.S. District Court here rovided information that raised questions, though less dramatic ones, about other prosecution testimony.

One government witness, William Fred Peters, had testified for example that he paid Flood a cash bribe at a private meeting in 1972 as the congressman sat in his Pennsylvania "command post" after Hurricane Agnes.

But three women who assisted Flood during his disaster relief efforts portrayed the command post yesterday as a chaotic, crowded place where the doors were always open and the possibility for private meetings and cash payoffs unlikely.

"Was Mr. Flood ever alone" at the command post, defense lawyer Axel Kleiboemer asked one of the women, Jean Connell.

"Oh my, no," she responded. "No way." There were always at least half dozen people in the room with the congressman, Connell, Mary Casterline and Jean Lukasiewicz said.

Flood's door "was always open," testified Lukasiewicz. "People were constantly running in and out."

Several witnesses have accused Flood of directly accepting bribes from people seeking his help in obtaining government favors. A housing developer, a trade school operator, a foundation president, a banker and a rabbi seeking federal grants were allegedly among those who paid off the 75-year-old congressman. Flood was chairman of the Appropriations subcommittee for Labor and Health, Education and Welfare until he stepped down from that post yesterday.

Even among various prosecution witnesses there was confusion and contradiction as to the dates and times the alleged bribes were paid. One of the better-documented instances was the alleged payoff Kevin Clark attempted to refute yesterday.

Robert Gennaro, the Pennsylvania housing developer, had testified that he delivered $2,000 cash to Flood at the congressman's home sometime on the evening of Oct. 31 and probably between 9:30 and 10 p.m.

He said Flood arrived at his home at that hour accompanied by Stephen B. Elko, then the congressman's administrative aide.

At that hour, Clark said yesterday, he was driving Flood on the campaign trail in northeastern Pennsylvania. At 9 p.m., he said, he and Flood were at a country club political rally miles away from the congressman's home.

At 9:15 p.m., they were on the road to another ally in Freeland, Pa., Clark testified, and at 10:15 p.m. they began the 25-mile ride back to Flood's home.

"We got there abut 11 p.m.," Clark said. And "no one but the housekeeper" was there when they arrived, he added. There was no sign of Elko and no sign of Gennaro, he testified.

Clark said he went inside Flood's home as he dropped off the congressman because "I had left something behind, some gloves or something."

Prosecutor Mark Tuohey then began his cross-examination, eliciting the acknowledgement that Clark's brother, Flood's press secretary, had on Monday of this week "discussed the events" of Oct. 31 with him.