Members of China's mostly Muslim Uighur minority and overseas groups on Thursday dismissed China's account of a Tiananmen Square "terrorist attack" as a dubious pretext for repression, amid signs of stepped-up security.

Beijing police said on Wednesday that Usmen Hasan -- in an SUV carrying his mother and wife, jihadist banners and machetes -- sped onto the pavement, crashed in front of a giant portrait of Mao Zedong and set the car alight.

The incident in the symbolic heart of the Chinese state killed two tourists, with 40 other people injured, and all three in the car died, police said.

Five other suspects with Uighur-sounding names were captured within 10 hours, although police only announced their detention two days later.

The Uighur minority is concentrated in China's far-western region of Xinjiang, where ethnic tensions and discontent with the government periodically burst out into violence.

Beijing regularly calls such incidents "terrorism", but Uighur organisations dismiss that as an excuse to justify religious and security restrictions. Information in the area is tightly controlled.

"I don't think there are any Uighur terrorist organisations, but China gives us a terrorist hat," said a Uighur at a university campus in the capital, who asked not to be named.

"I love this country but I'm afraid that people won't understand me," he added. "It's possible that some would take this kind of extreme measure, but because... they had a very sad experience."

He and other Uighurs around the capital described discrimination they had encountered. A chef in a Xinjiang restaurant declined to talk about terrorism for fear he would come "under pressure".

Alim Seytoff, a US-based spokesman for the overseas World Uyghur Congress (WUC), called the official narrative of the Tiananmen event full of holes and discriminatory.

"The Chinese claim is in a way very unbelievable, to some extent outrageous," he told AFP.

"The only reason this is labelled as a terrorist incident is because the passengers happened to be Uighurs."

Seytoff questioned why an attacker would kill his own family, and how religious material could survive in a car engulfed in flames.

"Why would he bring his mother and his wife?" Seytoff said.

"The car was burned almost to the ground, the three people were burned to death, and the flag wasn't burned -- in the car?"

Seytoff said there was a pattern of authorities labelling Uighurs as terrorists based on "thin evidence".

He dismissed claims of organised resistance in Xinjiang, describing incidents instead as "sporadic, individualistic, out of desperation".

According to Chinese state-run media a "terrorist attack" in the Turpan area in Xinjiang left 35 people dead in June, and 139 people have been arrested in recent months for propagating jihadist ideology.

Ethnic tensions have risen in Xinjiang since millions of members of China's Han majority moved to the resource-rich region, where they largely control the economy.

Rioting in the capital Urumqi involving both ethnic groups in 2009 left 200 people dead.

Seytoff warned Uighurs could face tighter repression after Monday's incident, particularly in the capital, where the WUC said 93 people have been rounded up.

In Xinjiang residents of Turpan said security had been ramped up, as it was after the June violence.

A restaurant manager surnamed Wang said police had alerted them to "prepare against attacks".

State-run media warned Thursday Uighurs would be the "biggest victims" of the Tiananmen incident.

Police had refrained from stating the attackers' ethnicity but the Global Times, which is close to the ruling Communist party and often strikes a nationalist tone, said that all those involved were Uighurs.

"People from Xinjiang, especially the Uighurs, will be the biggest victims," it said. "The ordinary work and study of Xinjiang people" in other parts of China "may be affected," it added, urging people in Xinjiang to "understand the negative effects and overcome them by cooperating".

It also exhorted Han Chinese to "make the Uighurs feel our sincerity".

Beijing's foreign ministry spokeswoman Hua Chunying condemned the Tiananmen incident Thursday as "an action against humanity, society and civilians".

It was "extremely wrong" to link the actions of "a small group of extremists with Chinese policy on ethnic groups and religion", she added.