Black South African nationalist Nelson Mandela spent Saturday undergoing tests and treatment for a lung ailment in a heavily guarded hospital.

The 70-year-old Mandela, jailed for life in 1964 for seeking to overthrow white-minority rule, was transferred from Pollsmoor prison to the Tygerberg Hospital in Cape Town on Friday.He was suffering from a "left-sided pleural effusion," or fluid in the membrane of the left lung, according to a terse medical bulletin.

Doctors, saying that Mandela was "comfortable and in a satisfactory condition," refused to disclose any details.

Armed police barred access to the fourth floor in the non-white wing of the huge hospital. The ward where Mandela was being treated was declared off-limits to most nurses, and reporters were ordered away from the building.

A doctor in another South African hospital told Reuters the symptoms described in the bulletin could be caused by an infection, tuberculosis or a malignancy.

Either way it could be very painful, said the doctor, who asked he not be identified. He said it was impossible at this stage to say how long Mandela would have to remain in hospital.

Before his sentencing, Mandela led the African National Congress in a low-level guerrilla campaign against white domination. Four years earlier the group was outlawed by the authorities.

Mandela, whom millions of South African blacks and some whites call their leader, has been kept at Pollsmoor despite an international clamor for his release.