Former South African president and anti-apartheid leader Nelson Mandela remains in a "serious but stable" condition in a Pretoria hospital after four days of treatment for a lung infection, the government has said.

A statement on Tuesday said President Jacob Zuma had been updated by doctors on the health of the 94-year-old, who became the country's first black president after historic all-race elections in 1994.

"President Zuma has full confidence in the medical team and is satisfied that they are doing their best to make Madiba better," it said, referring to Mandela by his clan name.

Police have tightened security around Pretoria's Medi-Clinic Heart Hospital which is treating Mandela.

Around a dozen police officers were deployed outside the building, which was cordoned off by barriers and police tape to keep the domestic and international reporters and television crews stationed outside away from the entrance.

All vehicles going into the building were being searched. 

Mandela was admitted early on Saturday with a recurring lung infection.

It is his fourth hospital stay since December, and there is a growing realisation among South Africa's 53 million people that they will one day have to say goodbye to the father of the "Rainbow Nation" that Mandela tried to forge from the apartheid-era.

Mandela has received visits from family members including his current and former wives, Graca Machel and Winnie Madikizela-Mandela.

Mandela has a history of lung problems dating back to his time on the Robben Island prison camp near Cape Town. 

Before his 1990 release he spent nearly three decades in prison for conspiring to overthrow the apartheid government.

Source: Agencies