Veteran South African activist Ahmed Kathrada has joined a growing chorus of voices calling for the resignation of President Jacob Zuma.

The 86-year-old, who was jailed with Nelson Mandela in the 1960s, asked Zuma to "submit to the will of the people" after a court ruled on Thursday that the president had violated the constitution.

"Today I appeal to our president to submit to the will of the people and resign," Kathrada wrote in an open letter published by local media.

"I know that if I were in the president's shoes, I would step down with immediate effect," Kathrada wrote.

Kathrada added that Zuma should be aware that his "outstanding contribution to the liberation struggle stands to be severely tarnished if the remainder of your term as president continues to be dogged by crises and a growing public loss of confidence in the ANC and government as a whole." He was referring to the ruling African National Congress party, of which Zuma is currently head and Kathrada is a longtime member.

Zuma defiant

South Africa's Constitutional Court found Zuma guilty of having used public funds to renovate his private home, a property valued at some $24 million (21 million euros) in 2014. He added a swimming pool and a cattle enclosure but wrote them off as necessary "security" measures.

The court ordered Zuma to repay $14.6 million to the public coffers.

After long denying that he had done anything wrong, Zuma apologized for the transgression, the latest in a series of scandals to hit his presidency. But he has so far refused to step down.

Zuma continues to face criticism over his close relationship with wealthy business magnates from the influential Gupta family, who are said to play an undue role in influencing policy.

The president's financial scandals are seen by some as a signal of how widespread corruption has become within the ANC, which has ruled since South Africa's first free elections in 1994.

A member of South Africa's ethnic-Indian population, Kathrada began working as a peace and anti-Apartheid activist as a teenager. In 1964, he was sentenced alongside Nelson Mandela, Govan Mbeki, and several others to life imprisonment for continuing the activities of the then-banned ANC.

Like Mandela, Kathrada spent his time in jail on the infamous Robben Island and was finally released in 1990. He later served as an adviser to Mandela during his presidency.

es/jm (AFP, dpa)