KHARTOUM (Reuters) - Around 20 people have been killed and dozens wounded in dawn attacks on a sit-in outside Sudan’s defense ministry by protesters calling for President Omar al-Bashir to step down, the head of the main opposition party said on Tuesday.

Veteran leader Sadiq al-Mahdi also called in a statement for “a select military command” to negotiate a transition toward democracy, following more than three months of protests that represent the most sustained challenge to Bashir’s 30-year rule.

Mahdi’s remarks came as several thousand protesters continued for a fourth day a sit-in outside the compound in central Khartoum housing the defense ministry, Bashir’s residence and the country’s security headquarters.

Early on Tuesday, Sudan’s National Intelligence and Security Service tried twice to disperse the protesters, breaking into the area using pickup trucks, witnesses said.

Security forces also tried to disperse the sit-in early on Monday, but witnesses and activists said soldiers moved to protect the protesters.

An opposition doctors’ committee put the death toll in Sudan since the sit-in began on Saturday at 21, including five soldiers, with more than 150 injured.

Amnesty International said in a statement that nine people had reportedly been killed in Sudan since Saturday, and it had verified two deaths on Tuesday, one of them outside the compound.

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The interior ministry could not immediately be reached for comment.

A senior leader of the Sudanese Professionals’ Association (SPA), the main anti-government protest organizer, called on Tuesday for the sit-in to continue and for similar protests “in front of all armed forces headquarters throughout Sudan”.

In the first public comments by a senior SPA leader, Omar Saleh Sennar also said his group wanted a civilian transitional government and would only negotiate with the army, considering it “the guarantor of the political system in Sudan”.

“I am now, from the leadership of the Professionals’ Association, present at the sit-in and more leaders of the association will subsequently appear to the public,” he told Reuters by phone.

ECONOMIC CRISIS

Since Dec. 19, Sudan has been rocked by persistent protests sparked by the government’s attempt to raise the price of bread, and an economic crisis that has included fuel and cash shortages.

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The protests escalated on Saturday, when activists, trying to persuade the country’s armed forces to side with them, marched toward the defense ministry compound.

Mahdi, leader of the main opposition Umma Party, said the march and sit-in - timed to coincide with the April 6 anniversary of a 1985 military coup that forced long-time autocrat Jaafar Nimeiri to step down after protests - were “unprecedented” and had received a “kind of hospitality from the armed forces”.

Mahdi was Sudan’s democratically elected prime minister when Bashir led his overthrow in 1989 in an Islamist-backed military coup.

On Monday, the interior ministry said 39 people, including three security forces personnel, had died since protests began on Dec. 19. A spokeswoman for SPA put the death toll at nearly 70.

Early on Tuesday, Reuters witnessed dozens of young men blocking Nile Street, a main road in the capital where ministries are located. The protesters used scrap metal barriers to block traffic on the street, which is located around one kilometer away from the presidential palace.

Soldiers on armored vehicles were guarding the presidential palace but not moving against demonstrators, a Reuters witness said.

Britain, the United States and Norway said Sudanese authorities must deliver a credible plan for a political transition or risk causing greater instability.

A Sudanese police spokesman was quoted by state news agency SUNA as saying police had been instructed “not to confront citizens and peaceful gatherings”.