A member of the 2,300-strong hospital staff told Reuters by telephone, "No one is allowed to enter or leave,"
"There are policemen and security guards standing outside."
The People's Hospital of Peking University was being disinfected and its patients and more than 2,000 employees moved to one of six hospitals in Beijing designated to handle SARS, the university said.
It did not say how many patients were involved, but the Web site for People's Hospital says it has 1,020 beds.
The hospital's front gate was closed and a sign said patients and employees were barred from leaving, as were all items used in the building.
Correspondents reported seeing visitors leaving bags of food and clothes at the facility's entrance, which a guard handed through the gate to masked hospital employees.
The quarantine began hours after Wednesday's World Health Organization announcement advising against travel to Beijing.
The hospital is not one of those set aside to treat SARS patients, but it has at least 60 confirmed or suspected cases among nurses and doctors, Reuters reported.
This quarantine is the latest move by a government that declared war on SARS last week, five months after the virus is believed to have first appeared in the southern Chinese province of Guangdong and started spreading. China has previously come under fire for being slow to react to the SARS outbreak.
Mainland China has reported just over half of the more than 4,200 infections the WHO reports worldwide.
Beijing, a city of 14 million people, has reported almost 775 SARS cases and 39 deaths and the number of infections is mounting by scores daily.
Anxious city residents cleared supermarket shelves of food on Thursday amid unease about possible shortages.
"You should have seen it. Lines at the cash registers stretched all the way to the store's back wall," a clerk at a Jingkelong Supermarket told the Associated Press.
Other East Asian nations were also working to contain the disease.
If any of the 2,500 Singapore residents under home quarantine go out in public, they will be sent to a medical camp that their government is preparing.
The island nation's parliament was also planning Thursday to amend public health laws to let authorities fine or imprison quarantine violators without going to court.
In Taiwan's capital Taipei, a hospital with a reported SARS outbreak was sealed off and all buildings within a half-mile radius from the hospital ordered disinfected. Taiwan also imposed some restrictions on visits from China.
Meanwhile, the World Bank said East Asian economies should be able to weather the disruption caused by SARS, although the virus could have severe short-term economic effects.