Five people including a Philippine tourist were killed and 38 were injured after a vehicle ploughed into crowds in Beijing's Tiananmen Square on Monday and caught fire, police said.

The blaze sent clouds of smoke billowing into the air near a giant portrait of Mao Zedong that hangs at one end of the square, the site of pro-democracy protests in 1989 which were brutally crushed by the authorities.

Witnesses and reports said the SUV vehicle drove along the pavement outside the Forbidden City, the former imperial palace, before crashing – prompting speculation the incident was intentional.

Immediately afterwards a security operation swung into force on the vast plaza, the symbolic centre of the Chinese state.

"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 near a portrait of communist China's founder that hangs on the Forbidden City's towering wall, while crowds looked on.

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

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

"The incident led to five deaths and 38 injuries," Beijing police said on their verified account on Sina Weibo, a Chinese equivalent of Twitter.

The driver of the vehicle and two passengers were killed, along with two tourists, one a woman from the Philippines and the other a man from Guangdong province in southern China, they said.

Three Philippine tourists and one Japanese were among the injured, police added, saying the vehicle had crashed into the guardrail on Jinshui Bridge, which crosses the moat around the Forbidden City, and then caught fire.

The Southern Metropolis Daily quoted an injured Philippine woman named Francesca as saying: "I heard the car's horn honking, but I noticed it too late. My mind went completely blank, and when I woke again I was completely on the ground."

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 generally kept under tight security, with both uniformed and plain-clothes personnel deployed. Many are equipped with fire extinguishers.

Social media users speculated that the car was crashed intentionally.

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

 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.

The writer of another post 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."

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

State broadcaster CCTV did not mention the incident on its flagship evening bulletin.

The first reports of the crash trickled online Monday afternoon from Chinese social media users at the scene.

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

Police stopped 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.

 Vehicles were later allowed back on to the main road in front of the Forbidden City, a world heritage site that sees 14 million visitors a year.

As well as the 1989 student demonstration, Tiananmen Square has been the scene of other protests.

In January 1982 a woman taxi driver who had been fined for failing to fulfill 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 but was immediately detained. (AFP)