BEIJING -- Chinese police are stepping up enforcement of visa restrictions for foreigners living in Beijing, in an apparent sign of the government's heightening anxiety about the Olympics next year.

The crackdown, which began in recent weeks, is targeting foreigners living or working in Beijing under expired or misused visas, according to people affected and others familiar with the recent efforts. The crackdown has involved police spot checks at homes and hotels, and resulted in fines and, in some cases, detention or deportation.

As part of the effort, people residing in compounds known to have foreign residents have been stopped by police or visited in their homes and asked to prove their legal status.

Chinese officials referred inquiries about the crackdown to the Ministry of Public Security, which didn't respond to repeated requests for comment this week.

A notice sent to American citizens in Beijing Tuesday by the U.S. Embassy said that the security ministry's push to ensure foreigners have valid visas is "in preparation for the Olympics." An earlier Embassy notice last month said new regulations had taken effect in July that would bar foreigners from converting one kind of visa, such as a tourist visa, to a work visa, among other measures.

There have been several recent detentions of American citizens for overstaying visas or other violations, according to the U.S. Embassy. Other foreigners illegally working as nannies or housekeepers have also been among those caught.
[Foreign Visitor Arrivals to China]

It isn't clear precisely why the Olympics, slated for next August, have prompted a crackdown now. But observers say the government is concerned about disruptions of the Games by terrorists or by groups it finds politically objectionable, such as the Falun Gong spiritual group.

A senior Chinese official was quoted in the state-run China Daily newspaper this month saying that while the overall security situation for the Olympics "remains stable, we still face the challenges of terrorism, separatism and extremism."

Chinese officials "are concerned that certain undesirables may have come into China and may be residing in the foreign community here, and that some of them may represent risks to the Olympics games," said Peter Humphrey, managing director of ChinaWhys Co., a Beijing-based risk-management firm. Mr. Humphrey said the government may also be motivated in part by a desire to improve tax collection by cracking down on foreigners working without permission.

The Beijing crackdown doesn't appear to affect foreigners who abide by their visa restrictions. But many foreigners enter China with visas for tourism or short-term business trips, then stay longer than the allowed period, sometimes working illegally.

Longstanding rules have stipulated that, if caught, such people can be fined $66 a day, up to a total of $661, and can be detained for as many as 30 days. Until recently, though, lax enforcement has meant that foreigners working illicitly have been able to stay undetected for years. Similarly, foreign residents have been urged to go to their local police stations to register immediately after entering the country; in the past, those that didn't weren't pursued by the authorities.

Li Shizhong, assistant manager of the customer-service center of Yosemite, an upscale residential community in Beijing, said police have visited her compound more frequently in the past few weeks to check on foreign residents' registrations. In particular, police seem to be targeting nannies and housekeepers in the compound from the Philippines who were working illegally. "These are all existing regulations, but they were not strictly implemented in the past," said Ms. Li. "Recently, the local police have enhanced the checks on the foreigners and Filipino [housekeepers]. The police just come and check anytime they like."

Some Olympics officials from other countries say they are experiencing greater difficulty getting multiple-entry visas to China for their employees. This could pose a problem for those needing to make inspection trips to mainland China, where the bulk of the sporting events are being held, and Hong Kong, which will host the equestrian events, said Dave Currie, chief of mission for the New Zealand Olympics team.

Beijing police are also inspecting schools and hotels to see if they are strictly complying with laws that require hotels to register their foreign guests.

A 31-year-old American kindergarten teacher at a Beijing private school, for example, was asked by school administrators to stay home for three days last month because the police were planning to come check all the foreign teachers. "If you come to work, [we'll] get seriously fined," she said her boss told her. She had been working with an unexpired tourist visa and was also stopped in front of her apartment twice by police who were checking the foreigners in her compound, and is now waiting for the school to process a proper working visa for her.

A reception-desk employee for the China World Hotel in Beijing said that, although the hotel always checked guests' visas and passports, the staff was called into two meetings recently about enhancing the registration system. Foreign visitors found with expired visas are turned away, while those with visas close to expiration are given warnings.

--Mei Fong, Kersten Zhang and Zhou Yang contributed to this article.

Write to Loretta Chao at loretta.chao@wsj.com