President Reagan's controversial proposal to sell $8.5 billion in airborne warning and control system (AWACS) radar planes and other sophisticated military hardware to Saudi Arabia ran into heavy weather at a first hearing on Capitol Hill yesterday.

Gen. George Keegan, the former chief of Air Force intelligence, described the weapons proposed for sale as a "powerful offensive system" that would leave Israel defenseless against an Arab tank attack launched through Jordan.

Just before Keegan's testimony to the House Appropriations subcommittee on foreign operations, Chairman Clarence D. Long (D-Md.) announced that 253 representatives are now co-sponsoring a resolution opposing the sale, 35 more than the number needed to block it in the House.

In the Senate, where the administration is more hopeful, leading opponent Bob Packwood (R-Ore.) said yesterday that 46 senators were co-sponsoring a similiar resolution.

Both houses must vote down the sale to kill it. Reagan summoned Packwood to the White House yesterday and asked him to delay seeking additional signatures, but Packwood said he told the president he "would not be deterred."

Some of the administration's defenders also spoke up yesterday. Sen. John G. Tower (R-Tex.), who had not taken any position, sent his colleagues on the Senate Armed Services Committee a letter supporting the sale and urging them to withhold judgment until the administration makes its full case on Capitol Hill next month. His letter was co-signed by Sen. John C. Stennis (D-Miss.), the committee's ranking minority member.

Robert G. Neumann, former ambassador to Saudi Arabia, told the House subcommittee yesterday that the "leaders of Saudi Arabia have put their eggs in the American basket" and that the sale would make them more disposed to U.S. peace initiatives in the region.

Neumann served just two months as ambassador to Saudi Arabia before resigning in a policy tiff with Secretary of State Alexander M. Haig Jr.

The administration intends to sell the Saudis five AWACS planes, 1,177 Sidewinder air-to-air missiles, and fuel tanks to add range to the 62 U.S.-built F15 jet fighters the Saudis already have on order.

In dollar terms, it is the largest arms sale package ever proposed, and it is also expected to provide the Reagan administration with its most difficult foreign policy confrontation to date on Capitol Hill.

Technically, the House Appropriations subcommittee hearing yesterday was informational only--the House Foreign Relations Committee has jurisdiction on the bill--but the tone of it bore witness to the depth of conviction on the issue.

Long said the sale would be tantamount to "lighting matches over the most explosive region in the world." Ranking minority member Jack F. Kemp (R-N.Y.) complained that selling AWACS to the Saudis would diminsh U.S. intelligence capabilities in the region because it would mean an end to the U.S. control over the four AWACS that currently patrol Saudi borders and oil fields.

But the most vehement opposition came from Keegan, who took special umbrage at claims made by leaders of both the Carter and Reagan administrations to the effect that the weapons package was bascially defensive in nature.

Noting that the best offense is a good defense, he outlined a scenario in which the Arab nations could "split Israel in two in one night" by launching a tank attack through the West Bank.

Israel would try to defend itself with its jet fighters, but, Keegan went on, the sophisticated "look down/shoot down" technology of the AWACS, which can spot plane movements more than 200 miles away, would enable the Saudis to "intercept and destroy the entire Israeli squadron of fighter bombers in 15 to 30 minutes."