South Africa held an official memorial service in Pretoria Thursday for anti-apartheid icon Albertina Sisulu.

She will be buried near her husband's grave on Saturday outside Johannesburg. Thousands of people are expected to travel from across South Africa to attend the funeral.

Albertina Sisulu, known as Ma Sisulu, died late Thursday at her home in Johannesburg. She was 92.

The Nelson Mandela Foundation issued a statement after her death saying “South Africa has lost a treasure.”

Sisulu was born in the Tsomo district of Eastern Cape in 1918 and moved to Johannesburg in 1940 to train as a nurse. There she met anti-apartheid leader Walter Sisulu. The two married in 1944, with former president Nelson Mandela serving as the best man.

When South Africa's apartheid government jailed Walter Sisulu and Mr. Mandela in 1964, she took up the cause. Sisulu continued to lead despite being jailed, tortured and banned from meeting with more than one person at a time.

In 1983, Sisulu formed the United Democratic Front, helping to revitalize the anti-apartheid movement.