After proceeding through a virtually flawless countdown to zero, an Ariane rocket -- the U.S. space shuttle program's main competitor -- failed to lift off from its launch pad tonight with a payload of U.S. and Brazilian satellites.

Frederick D'Allest, president of Ariane, said it would be at least a week before the next countdown. "We are pretty confident that we'll be able to launch in eight to 10 days," he told guests and officials in the Ariane control room.

The malfunction dealt a setback to Arianespace Inc., which hopes to overtake the United States in the commercial space race. It was also a blow to GTE Spacenet in McLean, Va., which owns one of the satellites aboard the rocket.

Launch officials said cryogenic arms that pump supercool hydrogen and oxygen into the rocket's third stage failed to obey a computer order to begin retracting at liftoff-minus-13 seconds. Five seconds later, computers in the launch firing room picked up a status report of the malfunction and automatically shut down the mission, preventing ignition and blastoff.

Scores of U.S. and foreign satellite executives flown here by Ariane and GTE Spacenet watched in befuddlement when the countdown hit zero and nothing happened.

The malfunction, D'Allest said, "was certainly a ground-equipment problem -- we have to see if it was in the switch or whether it was in the computer." To investigate, he said, fuel will have to be drained from all three stages of the 160-foot rocket, causing the delay.

"We know that scrubbing a launch date is part of the process," D'Allest said. "It is not the first or last postponement of an Ariane launch."

"Of course we're disappointed for it not to go," said Jerry Waylan, president of GTE Spacenet, whose new $90 million G-Star satellite was atop the rocket. "But it's not really a loss. We've got contingency plans . . . . It will be launched at some point."

Had the ignition on the rocket begun with the arms in place, it could have resulted in a major explosion, one official said. Other officials, however, said there were enough safety backups to prevent a catastrophe.

After a perfect launch only three weeks ago, Ariane was banking on another success tonight to capitalize on the grounding of the U.S. shuttle fleet because of the Jan. 28 Challenger explosion. In addition to the GTE telecommunications satellite, the rocket also carried a Brasilsat-2 satellite designed to route telephone calls, telex messages and television signals for Brazil.

Ariane was also testing a new launch pad here on the outskirts of the Amazon jungle that would enable the firm to launch satellites into orbit every four weeks, officials said.

With the U.S. shuttle grounded, Ariane officials have been deluged with new requests from satellite firms to book space on their unmanned rockets. The result, industry analysts contend, could be a multimillion-dollar windfall for the French that would pose longterm economic problems for the U.S. program, undercutting its oft-proclaimed goal of commercial viability.

NASA, from the shuttle's inception, sold it as a system that would ultimately "pay for itself" by carrying commercial payloads into orbit at prices theoretically pegged to full cost recovery. But in recent years Arianespace Inc., a privately owned concern spun off by the European Space Agency (ESA), began offering basically the same service on less glamorous unmanned rockets, while charging lower prices.

"It was a political decision in the U.S. to use the shuttle," Philippe Rasse, Ariane's director of engineering, said here in an interview. "But we realized that you don't have to launch automatic satellites with astronauts. It's cheaper without them and you don't have to take risks with the lives of someone. It just makes more sense this way."

NASA officials acknowledge that they may have put too much of a burden on the shuttle and, under a new policy announced this month by acting administrator William R. Graham, the agency will encourage the launching of commercial payloads on private unmanned rockets.

But some at NASA are skeptical about private ventures, pointing to the hefty subsidies Ariane received from ESA in its early days.

"There's no way these commercial rocket companies can compete against Ariane unless they're subsidized," said Chester Lee, NASA's director of customer services. "I think what will happen is the Japanese will start launching rockets, then they'll fight it out with Ariane and we'll be closed down. There's a lot of money in this business that's going to be flowing overseas."

But Ariane also has had dramatic failures. Last September, while French President Francois Mitterrand watched glumly in the control room, GTE lost an $80 million satellite when an Ariane rocket veered off course and had to be destroyed over the Atlantic.

Still, Ariane had been giving NASA stiff competition even before the Challenger explosion. The order book for 1986 to 1988 showed 21 reserved satellite flights on Ariane, including two by GTE, compared with 17 for the shuttle.

Then, with the Challenger disaster, the roof caved in. Two U.S. firms that had been booked on the shuttle signed "right of first refusal" contracts promising them the next available slot on Ariane. International Business Machines Corp. says it is "exploring" switching its next telecommunications satellite to the French. Intelsat, the international satellite consortium, recently asked Ariane to move up the launch of one of its already scheduled satellites from 1988 to 1987 to make up for the anticipated delay in shuttle launches.

D'Allest said at a news conference here earlier today that the company expects to make room for eight extra satellite slots on launches between now and 1988, a move that he estimated will bring the firm $250 million to $300 million in new revenues.

D'Allest and others with Ariane are touchy at being perceived as exploiting what they call "the tragic accident" of Challenger. Still, they are leaving little to chance. Executives from a host of satellite firms -- including American Satellite Co. in Rockville, Md., Ford Aerospace and the Japanese-owned Space Communications Corp. -- were flown here by Ariane for tonight's expected launch and treated to a whirlwind of parties and lavish feasts.

"This tropical setting is not so bad," said Masauki Kuzu, executive vice president of Space Communications. "And, of course, right now we only have one choice."