Police used batons to clear around 80 protesters who were blocking a major road in Khartoum on Friday to demand the release of Sudanese opposition leader Sadiq al-Mahdi, arrested last month.

Mahdi, a former prime minister who heads the opposition Umma Party, was detained after the public prosecutor ordered an investigation into charges that he insulted security forces in comments about a surge in violence in the Darfur region.

The protesters came out of a Khartoum mosque after Friday prayers, chanting: "The people want to change the regime," and "Freedom, peace and justice". They carried banners calling for Mahdi's release. Last week some 600 people gathered in protest.

Mahdi was prime minister in Sudan's last elected civilian government before he was overthrown in 1989 by the current president, Omar Hassan al-Bashir.

During his quarter-century in power, Bashir has weathered protests, armed insurgencies, US trade sanctions, the secession of oil-producing South Sudan and an arrest warrant - still pending - from the International Criminal Court on charges of masterminding genocide and other war crimes in Darfur.