Since early January, authorities have cracked down on the opposition and activists after they called for anti-government demonstrations to protest a surge in food prices, notably bread.

Sporadic protests have erupted since then in Khartoum and some other parts of Sudan that anti-riot police and security agents have so far swiftly managed to disperse.

Washington's mission in Khartoum expressed concern over the arrests of "hundreds of political leaders, activists and ordinary citizens".

Many of the detainees were being "held in inhumane and degrading conditions, and without access to lawyers or family", the embassy said in a statement.

"The United States believes in... the right to peacefully assemble without recrimination", it said.

Those detained include Fadlalla Burma Nasir, deputy head of Sudan's main opposition Umma party, Mokhtar al-Khatib, head of the Sudan Communist Party, and top activists Kamal Ismail and Mohamed al-Hafiz.

Several journalists covering the protests have also been detained, although most have been freed.

The embassy's remarks come at a time of improved relations between Washington and Khartoum after the United States on October 12 lifted a decades-old trade embargo it imposed on Sudan in 1997.

European embassies in Sudan have also urged Khartoum to release the detainees and to ensure they are "not mistreated".

Sudanese authorities are eager to prevent a repeat of deadly unrest that followed an earlier round of subsidy cuts in 2013.

Rights groups say dozens of people were killed when security forces crushed 2013 demonstrations, drawing international condemnation.

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