Bashir, who is wanted over alleged war crimes, crimes against humanity and genocide in the Darfur conflict, mostly travels to countries that have not joined the ICC, but South Africa is a signatory of the court's statutes.

The Pretoria High Court said in a statement it was "compelling respondents to prevent President Omar Al-Bashir from the leaving the country until an order is made in this Court."

The hearing is set to take place later Sunday, the opening day of African Union summit.

The ruling came after the Southern African Litigation Centre, a legal rights group, launched an urgent court application to force the authorities to arrest Bashir.

"South Africa has an obligation to arrest him," Johannesburg-based rights lawyer Gabriel Shumba told AFP.

"Failure to do so puts them in the same bracket as other African regimes who have no respect for human rights. It's actually a test for South Africa."

A South African government official at the summit confirmed media reports that Bashir was at the meeting, but declined to give further details.

"There's no point denying it now," said the official, speaking on condition of anonymity.

The summit is chaired by Zimbabwe's President Robert Mugabe, who has urged African leaders to pull out of the ICC, which critics accuse of targeting Africa.

The ICC said in a statement from its headquarters in The Hague that it "calls on South Africa … to spare no effort in ensuring the execution of the arrest warrants" against Bashir.

It said South Africa diplomats had been pressed last month to arrest Bashir if he attended the summit, but that they replied they faced "competing obligations" over the issue.

Darfur erupted into conflict in 2003 when insurgents mounted a campaign against Bashir's government, complaining their region was politically and economically marginalised.

'GRAVE CRIMES'

More than 300,000 people have been killed in the conflict and fighting has forced some 2.5 million people to flee their homes, the United Nations says.

Khartoum, however, disputes the figures, estimating the death toll at no more than 10,000.

"Allowing President al-Bashir into South Africa without arresting him would be a major stain on South Africa's reputation for promoting justice for grave crimes," said Elise Keppler of Human Rights Watch.

"South Africa's legal obligations as an ICC member mean cooperating in al-Bashir's arrest, not in his travel plans."

The two days of discussions among the 54 member states were set to focus on the political unrest in Burundi and the migration crisis across the continent.

Burundi has been plunged into instability by President Pierre Nkurunziza's push to run for a third five-year term.

Violent protests have left some 40 people dead and 100,000 people have fled the country, raising peace and security concerns in the region.

Mugabe and South African President Jacob Zuma are among those scheduled to speak on Sunday.

Also attending is Nigeria's newly-appointed President Muhammadu Buhari, whose country is battling the onslaught of Islamist group Boko Haram.

The leaders of two of Africa's other major economies, Egypt and Angola, are absent.

The summit comes two months after a wave of xenophobic violence swept parts of Johannesburg and Durban as African immigrants were hunted down and attacked by gangs.

At least seven people died in the unrest, which badly strained relations between South Africa and many countries in the region that complained at their citizens being attacked.

The subject of xenophobia was due to be tackled in a closed session on Sunday before the opening ceremony.

The session was also set to discuss migration as nearly 1,800 African and Middle Eastern migrants have died this year trying to cross the Mediterranean.