CAPE TOWN (AFP) - Archbishop Desmond Tutu, South Africa's anti-apartheid icon, Nobel Peace Prize laureate and a man dubbed the "moral compass of the nation", marks his 90th birthday on Thursday (Oct 7) and is set to make a rare public appearance.

The jovial Tutu, who does not hesitate to speak out against injustice even now, is due to attend a special thanksgiving service at St George's Cathedral in Cape Town, where he was appointed South Africa's first black Anglican archbishop.

Later, the archbishop emeritus and his wife Leah will spend the day at home with daughters Naomi and Mpho, grandchildren and great-grandchildren.

The day will culminate with an online lecture from the Dalai Lama, former United Nations rights chief Mary Robinson, activist and Mr Nelson Mandela's widow Graca Machel, and South Africa's former ombudswoman Thuli Madonsela, who is highly respected for her courageous exposure of corruption.

The line-up of speakers is a reminder of Archbishop Tutu's values, surrounding himself with rights advocates at a time when South Africa's current leaders are better known for lavish lifestyles and billion-dollar bank accounts.

The Tutu Trust said a "deluge of love and well-wishes" have been pouring in from individuals and organisations across the globe.

In a congratulatory message, President Cyril Ramaphosa lauded Archbishop Tutu "as a fighter in the cause for human rights, for equality and for social justice in the 59 years since his ordination".

He paid tribute to "The Arch" as he is fondly referred to in South Africa "for a life that has been well-lived in honesty, integrity, fearlessness and service to humanity."

Nelson Mandela Foundation chief executive Sello Hatang said: "The Arch is an extraordinary human being. A thinker. A leader. A shepherd."

A tireless activist, Archbishop Tutu has in recent years slammed even the ruling African National Congress party, which fought tirelessly against white minority rule, for cronyism and nepotism after apartheid ended in 1994.

In the past, he confronted homophobia in the Anglican Church, challenged the late Mr Mandela over generous salaries for Cabinet ministers and stridently criticised the endemic corruption that mushroomed under former president Jacob Zuma.

"At times when we have found ourselves losing our way, you have taken us well to task," said Mr Ramaphosa.

"For nearly three decades, yours has been a voice of conscience, guiding us and motivating us to do better by our people," he added.

Ordained at the age of 30 and appointed archbishop in 1986, Mr Tutu lobbied for international sanctions against apartheid, and later for human rights on a global scale.

He turned his focus on the thorny issue of reconciliation in the post-apartheid era as head of the Truth and Reconciliation Commission.

He retired in 2010 and rarely speaks in public now.

He was last seen in public in May, when he and his wife got their Covid-19 vaccinations.

Archbishop Tutu was diagnosed with prostate cancer in 1997 and has undergone repeated treatment.