Fireworks at the climax are a highlight of any opening ceremony and they were too at the Asian Games in China, except for one thing -- they weren't real.

The computer-generated pyrotechnics fooled television audiences and disappointed onlookers after host city Hangzhou opted for a virtual display, citing environmental concerns.

Chinese President Xi Jinping declared the 19th Asiad open at an extravaganza packed with foreign dignitaries on Saturday, but organisers ditched the traditional firework display.

Instead, they superimposed virtual "fireworks" that appeared to go off all over the city on the official live broadcast.

The decision left people who had gathered near the stadium scratching their heads at the anti-climactic finale, with comical videos from the night going viral on Chinese social media site Weibo.

In one video, an excited group of local residents aim their phone cameras at the lotus-shaped Hangzhou Olympic stadium, counting down in unison.

"Ten, nine, eight, seven, six, five, four, three, two, one... nothing?" voices can be heard saying in the video, before the crowd erupts into awkward laughter.

Another clip taken across the river from the stadium pans back and forth between the empty sky over the "Big Lotus" and the exuberant virtual firework display shown by state broadcaster CCTV on someone's phone.

The cameraperson shakes and shrieks with laughter, asking "what did we wait for?"

The virtual display was so convincing that some journalists were fooled by what they saw on TV, with one regional newspaper reporting that fireworks erupted around Hangzhou at the end of the ceremony.

Organisers had previously announced there would be no fireworks, citing the lower carbon footprint of a digital show.

"The opening ceremony of the Hangzhou Asian Games will break the tradition of having a fireworks performance, as we are sticking to a green philosophy in hosting the event," opening ceremony director Sha Xiaolan said.

tjx/pst