Former South African President Nelson Mandela's grandson has said Palestinians' lives under Israeli occupation are worse than anything Black South Africans experienced under apartheid, and urged the global community to come to their aid.

Mandla Mandela, 51, spoke to Reuters on Wednesday evening at Johannesburg Airport, where he was boarding a flight to Tunisia to join a flotilla aiming to deliver food and humanitarian supplies to Gaza despite an Israeli naval blockade.

"Many of us that have visited the occupied territories in Palestine have only come back with one conclusion: that the Palestinians are experiencing a far worse form of apartheid than we ever experienced," Mandela said.

"We believe that the global community has to continue supporting the Palestinians, just as they stood side-by-side with us."

Israel rejects comparisons between the lives of Palestinians who have lived under occupation or economic blockade for more than half a century and the apartheid era in South Africa, when the Black majority was ruled by a repressive white minority government.

It has also defended its tight restriction of the supply of humanitarian and other goods to Gaza, saying it aims to stop weapons from reaching the militant group Hamas.

Starvation is widespread in the enclave, according to the World Food Programme, and an authoritative hunger monitor says a quarter of the population are suffering from famine.

Mandela is joining a group of 10 South African activists in the Global Sumud Flotilla, which includes dozens of boats and hundreds of people from 44 countries including Swedish activist Greta Thunberg.

South Africa's African National Congress said their mission "echoes our own struggle for liberation".

Mandela emphasised that, when apartheid ended in 1994, it was after intense pressure and sanctions from other nations.

"They isolated apartheid South Africa and finally collapsed it. We believe that the time has come for that to be done for the Palestinians," he said.