Fans at a soccer match in Sudan's capital chanted slogans demanding President Omar Bashir step down and later clashed with police in the Khartoum suburb of Omdurman, according to activists and video clips posted online.
The clips show the fans in the stadium chanting "The people want to bring down the regime," one of the main slogans of the Arab Spring uprisings of 2010 and 2011. They also chanted "Freedom!"
The video clips show that not all the fans were taking part in the chanting.
Activists said police used tear gas and rubber bullets to disperse the fans after the match, in which local club al-Hilal beat a Tunisian side 1-0 in the African Champions League. Two of al-Hilal's all-time greats - retired players Haytham Mustafa and Faisal al-Agab - had called on fans to protest Bashir's rule during the match.
"Let the television networks hear your chants against the government. Go out and march to make the nation victorious and rewrite history again," al-Agab tweeted before the match. "Thank you al-Hilal fans, you have really shown that you are the true sons of the club of patriotism and freedom," Mustafa tweeted after the match.
In another tweet, al-Agab called on the fans of another Sudanese soccer powerhouse, al-Mareekh, to protest during and after a match scheduled for later on Monday in Khartoum.
The activists said there were casualties among protesters when they clashed with police after Sunday's match, but they had no exact figures.
The violence was the latest in a series of anti-government protests across Sudan, which were initially sparked by rising prices and shortages of food and fuel, but soon turned to demands for Bashir to step down. There were no reports by Monday afternoon of fresh protests, but news has been traveling slowly because of internet disruptions engineered by authorities.
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Bashir, in his mid-70s, seized power in a 1989 military coup that overthrew an elected but ineffective government. He is wanted by the International Criminal Court for committing crimes against humanity and genocide in the western Darfur region.
An opposition leader said 22 protesters have been killed in clashes with police since the unrest began last week. The government, which has imposed a near-total news blackout on the protests, has acknowledged fatalities but gave no figures.
Sudan's doctors, meanwhile, began an indefinite strike on Monday, with organisers reporting a widespread response. The strike is mainly focused on government hospitals, but the organisers said some doctors in private clinics were joining in.
The union of pharmacists said they would start an indefinite strike on Tuesday.
An umbrella coalition of professional unions said the doctors' strike would be the first of a series of work stoppages to force Bashir to step down. It said its members will march Tuesday on Khartoum's Republican Palace and submit a written demand for the president to step down. It also called on non-members to join in.
"Together we can rebuild everything and climb the ladder to progress and civilisation," said a statement by the coalition, an independent entity which operates outside parallel groups loyal to the government. "Let us all unite. Come and join so we can realise the will of the people."
One of Sudan's largest political groups, the Democratic Unionist Party, expressed its support for Tuesday's march on the presidential palace and called on its supporters to join in. The party enjoys wide support in eastern and northern Sudan.
Seeking to staunch the unrest, authorities have arrested more than a dozen opposition leaders and suspended university and school classes in much of Sudan. They have also tightened censorship of newspapers and sought to stop journalists from covering the street protests.
Bashir received a lifeline from the military on Sunday, which assured him of its loyalty and said it will continue to protect the nation in coordination with the police and the country's feared security agencies.
Participants in the protests have so far numbered in the hundreds or low thousands in each location, but their continuation for nearly a week despite the use of force by police and the escalation by unions suggest a level of popular discontent that Bashir will find difficult to control.
A career soldier, Bashir seized power with Islamists in a 1989 military coup. He has ordered the use of force against protesters in the past — including in the last round of unrest in January - successfully crushing them to remain one of the longest-serving leaders in the region.
Although his time in power has seen one crisis after another, he is seeking a new term in office. Lawmakers loyal to him are already campaigning for constitutional amendments that would allow him to run in the 2020 election.