By Tim Hepher and Brenda Goh

ZHUHAI, China (Reuters) - China showed its Chengdu J-20 stealth fighter in public for the first time on Tuesday, opening the country's biggest meeting of aircraft makers and buyers with a show of its military clout.

Airshow China, in the southern city of Zhuhai, offers Beijing an opportunity to demonstrate its ambitions in civil aerospace and to underline its growing capability in defense. China is set to overtake the U.S. as the world's top aviation market in the next decade.

Two J-20 jets, Zhuhai's headline act, swept over dignitaries, hundreds of spectators and industry executives gathered at the show's opening ceremony in a flypast that barely exceeded a minute, generating a deafening roar that was met with gasps and applause and set off car alarms in a parking lot.

Experts say China has been refining designs for the J-20, first glimpsed by planespotters in 2010, in the hope of narrowing a military technology gap with the United States. President Xi Jinping has pushed to toughen the armed forces as China takes a more assertive stance in Asia, particularly in the South China and East China seas.

"It is clearly a big step forward in Chinese combat capability," said Bradley Perrett of Aviation Week, a veteran China watcher.

State-owned Commercial Aircraft Corporation of China (COMAC) was also bullish on China's appetite for new civilian planes, estimating the market would need 6,865 new aircraft worth $930 billion over the next 20 years.

The COMAC forecast - similar to long-term outlooks from well-established rivals Boeing Co and Airbus Group - said China would make up almost a fifth of global demand for close to 40,000 planes over the next two decades.

QUESTIONS UNANSWERED

After screeching onto the Zhuhai stage as a pair at low-level, one of the J-20s quickly disappeared over the horizon, leaving the other to perform a series of turns, revealing its delta wing shape against bright sub-tropical haze.

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It was China's second successive display of stealth at the biennial show, following the 2014 debut of the J-31.

But analysts said the brief and relatively cautious J-20 routine - the pilots did not open weapon bay doors, or perform low-speed passes - answered few questions.

"I think we learned very little. We learned it is very loud. But we can't tell what type of engine it has, or very much about the mobility," said Greg Waldron, Asia Managing Editor of FlightGlobal. "Most importantly, we didn't learn much about its radar cross-section."

A key question whether the new Chinese fighter can match the radar-evading properties of the Lockheed Martin F-22 Raptor air-to-air combat jet, or the latest strike jet in the U.S. arsenal, Lockheed's F-35. The F-22, developed for the U.S. Air Force, is the J-20's closest lookalike.

But the mere display of such a newly developed aircraft was a revealing signal, others said.

"It's a change of tactics for the Chinese to publicly show off weapons that aren't in full squadron service yet," said Sam Roggeveen, a senior fellow at the Sydney-based Lowy Institute, "and demonstrates a lot of confidence in the capability, and also a lot of pride."

C919 PASSENGER JET ABSENT

Other aircraft on display on Tuesday alongside the latest Chinese weapon systems, radar and drones, included the Xian Y-20 strategic airlifter, and what organizers say is the largest amphibious plane now in production - the AG600.

The flying boat is officially promoted as a fire-fighting or search and rescue plane. But analysts note the AG600 - first unveiled 10 days after a Hague tribunal ruled against China's claim to parts of the South China Sea in July - is well suited to resupplying military outposts in the disputed area.

A model of a wide-body jet being developed by COMAC and Russia's UAC was also on show, revealing design details such as wingspan and cruising speeds for the first time. Airbus and Boeing dominate the wide-body segment.

Notably absent from the airshow schedule, though, was the 150-seater COMAC C919 passenger jet, which has been beset by delays and is now running three years behind original plans.

COMAC said at the show that China Eastern Airlines will be the launch customer for the C919, which may take its first test flight later this year or early 2017, and that it had clinched 23 new orders for its C919, taking total firm orders to 570.

(Additional reporting by Michael Martina in BEIJING; Editing by Kenneth Maxwell)