* Mandela remains in "serious but stable" condition
* Police presence beefed up at Pretoria hospital
* Anti-apartheid leader being treated for lung infection
Former South African president and anti-apartheid leader Nelson Mandela remained in a "serious but stable" condition in a Pretoria hospital on Tuesday after four days of treatment for a lung infection, the government said.
A statement said President Jacob Zuma had been updated by doctors on the health of the 94-year-old, who became South Africa'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 tightened security around Pretoria's Medi-Clinic Heart Hospital which is treating Mandela, revered around the world for leading the struggle to end South Africa's apartheid system of racial segregation and white-minority rule.
Around a dozen police officers were deployed outside the building, which was cordoned off by barriers and police tape to keep a phalanx of domestic and international reporters and television crews from the entrance.
All vehicles going into the building were being searched.
Mandela was admitted in the early hours of 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 ashes of apartheid.
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 wind-swept 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.