Security forces opened fire on Sudanese protesters Friday, witnesses said, as thousands marched through the streets of the capital in an opposition push to turn a wave of popular anger over fuel price hikes into an outright uprising against the 24-year rule of President Omar Hassan al-Bashir.

At least 50 people have been killed this week in the security forces’ crackdown on a startling burst of protests, sparked by cuts on fuel and gas subsidies. The marches are turning into the heaviest domestic challenge yet faced by Bashir, who has so far been spared the sort of anti-authoritarian popular revolts seen around the Arab world in the past two years.

Although he has kept his grip on the regime, Bashir has been increasingly beleaguered. The economy has worsened, especially since South Sudan broke away and became an independent state in 2011, taking Sudan’s main oil-producing territory. Armed secessionist groups operate in several parts of the country. And Bashir, who came to power as head of a military-Islamist regime after a 1989 coup, is wanted by the International Criminal Court over alleged crimes in Sudan’s western region of Darfur.

Protesters marched in several parts of Khartoum and in at least one other city, Wad Madani, after weekly Muslim prayers Friday. Security forces opened fire on marches on Street 60 in eastern Khartoum and Street 40 in the Omdurman district, witnesses said, speaking on the condition of anonymity for security reasons. At least one protester was shot to death, a doctor said, also declining to be identified because of the tense security situation.

In Omdurman, an opposition stronghold, the prominent opposition leader Sadiq al-Mahdi delivered the Friday sermon at a mosque, telling worshipers that Bashir has been spending the state’s budget on “consolidating power.”

“Life became unbearable. Citizens’ main concern is survival after the government gave up on its responsibility to provide subsidies,” said Mahdi, of the National Umma Party. “We call for changing the regime.”

After the sermon, a crowd of protesters marched from the mosque through the district, chanting, “The people want the downfall of the regime,” the slogan heard in Arab Spring uprisings from Tunisia and Egypt to Syria and Yemen.

Interior Minister Ibrahim Mahmoud said Friday that 600 people have been arrested this week for sabotage and will stand trial, according to the official Sudan News Agency.

The subsidy cuts come amid International Monetary Fund pressure on Sudan to curb spending and repay debts. Similar IMF-backed austerity measures announced last year also sparked protests that were quickly put down. Bashir justified the new measures, saying they would rescue the country from “collapse.”

Two rights groups, Amnesty International and the African Center for Justice and Peace Studies, accused the government of using a “shoot to kill” policy against this week’s protests, saying they had documented 50 deaths in rioting on Tuesday and Wednesday.

— Associated Press