JOHANNESBURG (Reuters) - South African anti-apartheid veteran Winnie Madikizela-Mandela, ex-wife of the late president Nelson Mandela, is "absolutely fine" after being admitted to hospital for observation, her spokesman said on Friday.

Doctors decided on Wednesday to keep the 80-year-old former first lady in hospital after she underwent a routine checkup.

"I was with her and she's absolutely fine," spokesman Victor Dlamini told television broadcaster eNCA.

Madikizela-Mandela spent several weeks in hospital last year for back surgery.

During her ex-husband's 27-year incarceration for his fight against apartheid, Madikizela-Mandela campaigned for his release and for the rights of black South Africans, undergoing detention, banishment and arrest.

(Reporting by Tiisetso Motsoeneng; Editing by Gareth Jones)