BEIJING – An SUV car crashed into a crowd in Beijing's Tiananmen Square Monday and burst into flames, killing three people and sending a tower of smoke billowing near a giant portrait of Mao Zedong.

Immediately after the incident a security operation went into effect on the vast square next to the Forbidden City, where pro-democracy protests in 1989 were brutally crushed by the authorities.

"I saw a car turn a bend and suddenly it was driving on the pavement, it happened fast but looked like it knocked people over," one eyewitness, who did not want to be named, told AFP.

"I heard an explosion and saw fire. The scene was very frightening," he added. "There were paramilitary police who told people to get back into their cars and stop taking pictures."

Images posted on Chinese social media sites showed the blazing shell of the SUV and a plume of black smoke rising close to a portrait of China's founder that hangs on the towering wall of the former imperial palace, while crowds looked on.

Several pictures posted online were deleted within minutes, streets leading to the square were blocked off and screens were erected.

Two AFP reporters were temporarily detained close to the site, with images deleted from their digital equipment.

"A jeep crashed into the guardrail on Jinshui Bridge, then caught fire," Beijing police said on their verified social media account. The Jinshui Bridge passes over the moat around the Forbidden City.

"It is confirmed that the jeep driver and the two other people in the car are dead," the statement said, adding the fire had been put out.

The official Xinhua news agency said 11 people – tourists and police officers – were injured and had been taken to nearby hospitals for treatment.

One 58-year-old Italian tourist said he was touring the Forbidden City when officers came in around noon and ushered everybody out.

Tiananmen Square is the symbolic centre of the Chinese state and is generally kept under tight security, with both uniformed and plain-clothes personnel deployed, many of them equipped with fire extinguishers.

Details on a motive were not immediately available, but social media users speculated that it could have been intentional.

"Is this the 2013 Tiananmen self-immolation incident?" asked one poster. "There's still a person inside the car!"

Another pointed out that the vehicle had driven through a pedestrian area, adding: "It couldn't have been a car accident but was a premeditated event."

Around 120 Tibetans have set themselves alight since February 2009 in Tibet itself and adjoining regions of China, in protest against what they see as oppression by Beijing.

Foreign ministry spokeswoman Hua Chunying said she did not know "the specifics" when asked whether there was any evidence of a terrorist attack, or any foreign casualties.

News of the incident first trickled online Monday afternoon in reports from Chinese social media users at the scene.

Pictures they posted showed the flaming wreck surrounded by several police and emergency vehicles, directly in front of the sign on the Tiananmen gate reading: "May the great unity of the world's people last for 10,000 years."

Soon afterwards police erected high curtain-like barricades directly in front of the Mao portrait, blocking passers-by from viewing the scene.

Police blocked people from entering the square and said an "event" was happening there, while an officer in a van blared orders to leave through a loudspeaker.

The main road in front of the Forbidden City was later re-opened to vehicles but no pedestrians were allowed near the scene of the incident.

Tiananmen Square is surrounded by several of China's key buildings and institutions, with Mao's mausoleum on the south side, the Great Hall of the People to the west, and China's national museum to the east.

The imperial Forbidden City, a world heritage site that sees 14 million visitors a year, is on the north side, with the Tiananmen Gate as its entrance, where Mao proclaimed the foundation of the People's Republic of China in 1949.

As well as the 1989 student demonstrations, it has been the scene of other protests in the past.

In January 1982, a woman taxi driver who had been fined for failing to fulfil her quota of fares drove her vehicle into a crowd at the Jinshui bridge, killing five people and injuring 19 more. She was executed 20 days later.

State media said in 2001 that five members of the Falungong religious sect had set themselves on fire on the square, but the group accused authorities of staging the incident.

In May 2007 a man from Xinjiang, the far western region home to Muslim Uighurs, tried to set fire to the Mao portrait and was immediately detained.