Tens of thousands of demonstrators marched through Cape Town on Saturday to protest Israel's military assault on Palestinians in Gaza, in one of the biggest rallies in the city since the end of apartheid.

Demonstrators carried posters stating "Israel is an apartheid state" and "Stop Israeli murder".

Criticism of Israel is not new in South Africa, and the former has previously expressed its outrage at comparisons of its policies towards Palestinians with the racist apartheid system.

"We estimate there are well over 30,000 people, and the figure could be as high as 50,000," Cape Town's executive director for safety and security Richard Bosman told the national Sapa news agency.

Chanting "Free Palestine" -- with some demonstrators wearing T-shirts reading "Africa understands colonialism" -- the protesters marched through the centre of the city to the national parliament.

The march was called by the National Coalition for Palestine, which groups more than 30 religious and civil society organisations, trade unions and political parties.

Among them are the youth league of the ruling African National Congress and its alliance partner the Congress of South African Trade Unions, as well as the Muslim Judicial Council.

The group said it was demanding "decisive action from the South African government against the Israeli attacks, killings, displacement and destruction of the Gaza Strip".

There have been calls from several organisations for Pretoria to expel the Israeli ambassador.

Fighting between Israel and Hamas has now killed at least 1,900 Palestinians and 67 people on the Israeli side, almost all soldiers, since July 8.

The United Nations says at least 1,354 of the Palestinians killed since July 8 were civilians, including 447 children.