Foreign embassies in China issued a warning Thursday about possible threats against "Westerners" in a popular Beijing neighborhood ahead of the Christmas holiday.
The notice, from the US and British governments, covered the popular shopping and nightlife area of Sanlitun.
The area has been the site of occasional violence against foreigners, including a grisly murder in August when a Chinese attacker stabbed a French man and his Chinese wife with a sword. The woman later died from her injuries.
Mass attacks in Beijing are very rare, although in October 2013 five people were killed and dozens injured when a vehicle ploughed into crowds in Tiananmen Square, with the suspects appearing to be mostly from the restive Xinjiang region.
Beijing described the incident as a terrorist attack.
The US embassy's statement on its website Thursday urged citizens to be vigilant, while the British embassy made a similar warning, saying it had received information about possible threats against Westerners on or around Christmas.
The French embassy passed on the US warning to its citizens.
Photos on social media showed a heightened police presence in front of a Sanlitun shopping center, home to major international brands.
They also showed a camouflage van surrounded by men armed with rifles wearing fatigues in a plaza near one of Beijing's two embassy districts.
The city's police department issued a yellow alert Thursday, the second level in a four-tier system, according to a post on its social media account.
A yellow alert is issued when authorities have information about a significant and approaching threat to public security, state media said. They can relate to natural disasters as well as issues linked to social order.
The alert, expected to last through New Year, would "ensure good social order during the upcoming holidays", police said, adding that it was due to "the gradual increase of shopping promotions, celebrations and entertainment activities."
It did not offer any more details, but the measure comes after a stampede in Shanghai last New Year's Eve that left over 30 dead and was blamed on poor crowd control.