Sudan's former prime minister and top opposition figure Sadiq al-Mahdi died from a coronavirus infection on Thursday, his party said.

The 84-year-old was the country's last democratically elected prime minister before he was toppled in 1989 by now-ousted president Omar al-Bashir in an Islamist-backed military coup.

The leader of the moderate Islamist National Umma Party was transferred to the United Arab Emirates for treatment three weeks ago after being hospitalised in Sudan and testing positive for Covid-19.

"We offer our condolences to the Sudanese people over his death," the party said in a statement. 

Mahdi was a staunch opposition figure during Bashir's long rule and threw his weight behind a mass-protest movement that eventually prompted the military to overthrow the president last year.

Since his ouster, Bashir has been jailed in Khartoum's high-security Kober prison, and was found guilty last December of corruption.

He is currently on trial in Khartoum for his role in the 1989 coup that brought him to power and the International Criminal Court (ICC) has charged him for alleged war crimes in the Darfur war that began in 2003.

If convicted, Bashir and his co-accused -- including former top officials -- could face the death penalty.

Sudan has so far recorded nearly 17,000 coronavirus cases including more than 1,200 deaths.