President Reagan's embattled plan to sell sophisticated radar surveillance planes to Saudi Arabia was dealt a serious blow yesterday when Senate Minority Leader Robert C. Byrd (D-W.Va.) declared that he will vote against the $8.5 billion deal.

"I do not believe this sale serves the best interests of the United States," Byrd said in a lengthy statement. His decision was hailed by Sen. Alan Cranston (D-Calif.), a leader of the opponents, as "a crucial turning point" in the battle over the Airborne Warning and Control System (AWACS) planes.

Since a big majority of Senate Democrats already had been expected to vote against the sale, Byrd's long-awaited decision was important mainly for its impact on potential waverers. Several Democrats, particularly from the South, have come under heavy administration pressure to switch sides. It was felt that Byrd could take some of these with him if he went to the president's side.

Many congressional sources predicted that Byrd's backing will encourage them to remain in the opposition when the Senate votes Wednesday on a resolution to disapprove the sale.

In an assessment that was generally shared by impartial head-counters on Capitol Hill, Cranston said 55 senators are now leaning against the sale, with 49 of them regarded as "firm" in their opposition. He added that Reagan appears to have 41 senators on his side, with the remainder uncommitted.

Although some sources offered counts that put the opposition at 54 or 53, there seemed to be wide agreement that the arithmetic demonstrates clearly that the administration faces an increasingly steep climb in the time remaining. To muster a majority, they pointed out, the administration must pry away at least five of those now leaning against, and win virtually all the uncommitteds.

Last week, the House voted 301 to 111 against the sale. If the Senate also votes against it, the deal will be blocked. That would hand Reagan his first major congressional defeat, and it would come on a matter that the president contends is vital to the success of his long-range strategic and diplomatic goals in the Middle East.

In Cancun, Mexico, where Reagan is attending a 22-nation economic summit, deputy White House press secretary Larry Speakes said Byrd's opposition was "not unexpected." Speakes added, "We remain optimistic that we have sufficient votes . . . . We are gaining ground."

While in Cancun, Reagan will meet with Saudi Arabia's Crown Prince Fahd. Despite administration denials, there is persistent speculation in Congress that the Saudis might make new concessions on control of the AWACS planes in hopes of overcoming Senate objections to the deal. Administration officials consistently have thrown cold water on such speculation and have said only that Reagan intends to keep lobbying senators personally right up to the time of the vote.

Byrd, who earlier had refused to reveal his intentions to anyone, went to such extraordinary lengths to keep his decision secret that he had his staff prepare two AWACS speeches, one for the sale and one against it.

He based his opposition on the contention that "the central issue for American policy in the Middle East is the Arab-Israeli dispute, and not the Soviet threat to the region," as the administration has said in arguing that the Saudis need the aircraft to protect their oil fields from external threats.

"As long as the Arab-Israeli dispute is pushed into the background, this sale does not make any sense," Byrd said. "Rather than contributing to stability in the region, I fear it will only raise the threshold of tension."

Cranston also dismissed the text of a letter of reassurance from Reagan circulating on Capitol Hill as a "wishy-washy document" that will "not convince anyone to vote for the AWACS sale." Administration sources said the text seen by various senators was "an early draft" circulated for comments, and is likely to be revised considerably before being submitted formally to the Senate.

The letter, intended as Reagan's written reassurances that the AWACS planes will not be misused, describes four "arrangements" about security and data sharing that would be incorporated into written agreements with the Saudis. However, the letter contains nothing that had not been revealed to the Senate in public testimony by administration officials.