(CNN) -- Riot police sprayed tear gas at hundreds of protesters who attempted to lead a march after Friday prayers from the two main opposition party mosques in Sudan's capital of Khartoum and its second city, Omdurman.
About 500 riot police surrounded a mosque and fired tear gas canisters at the crowd as they began to leave, witnesses said. Protesters responded by hurling rocks at officers.
The head of the opposition Umma party -- also the last democratically elected prime minister of Sudan -- was escorted to safety out of the mosque by supporters after the clashes began.
The two mosques in both cities are attended by supporters of Sudan's two largest opposition parties, the Umma and Democratic Union parties.
The clashes follow calls by Sudanese activists for mass protests as world leaders condemned authorities for cracking down on demonstrators. Thousands of people are demonstrating on the streets of Khartoum and Omdurman, demanding the president's ouster over economic woes.
Activists say that hundreds are being detained as the clampdown intensifies. They dubbed Friday's protests "elbow licking," a spoof on a phrase by the administration. An aide to the nation's leader once said that toppling the president is like trying to lick an elbow -- impossible
The demonstrations, which started June 16 at universities in the capital of Khartoum, have spread to other cities nationwide.
Protesters seeking to end President Omar al-Bashir's nearly 23-year rule have seized on recent budget cuts and tax increases to demand his ouster.
Police have fired tear gas and rubber bullets on protesters over the past two weeks after the government ordered a crackdown, witnesses said.
A spokesman for activist group Sudan Change Now, who goes by the name Ahmed Samir for security reasons, accused the government of using gangs of plain clothes thugs, known as "Rabatta," to beat protesters and reinforce the security forces.
"In Hajj Yousif district, the rabatta militias were firing AK-47's into the air. It is an attempt to intimidate but the government is in denial," he said.
"The demands of the people are clear -- they want a revolution to sweep away this government. The austerity measures we are suffering under are a result of this government's prioritizing of military and security. They want the people to pay the price for their irrational wars."
Authorities have arrested opposition leaders and forced some to sign paperwork vowing not to participate in any more protests, the United Nations said this week.
In addition, police seized injured protesters from hospitals and detained them, according to the United Nations.
"Tear gas, rubber bullets, live ammunition and other heavy-handed suppression will not resolve the frustrations and grievances of the people regarding shortcomings in their enjoyment of economic, social, civil and political rights," U.N. human rights chief Navi Pillay said Thursday.
"Genuine dialogue with government critics is far more effective than arbitrary detention and violence if the government wishes to create a stable and successful society."
U.S. officials also condemned the clampdown and arrests.
"A government that respects the rights of its citizens to freedoms of expression and peaceful assembly in order to raise their grievances does not respond to such protests by using unnecessary force," said Victoria Nuland, a State Department spokeswoman.
Government officials were not immediately available for comment.
Growing numbers of protesters have rallied in the capital with some demonstrations turning violent as crowds burned police trucks, cut off roads and hurled rocks at authorities.
In return, police have fired rubber bullets and struck protesters with batons , witnesses said.
"Leave, Bashir, leave!" protesters chanted. "Khartoum, please revolt against humiliation and dictatorship."
Al-Bashir has played down the protests and urged citizens to understand the new austerity measures, which lifted fuel subsidies and cut the Cabinet by half to reduce expenses. In a televised speech on state media this month, he said the measures are necessary to help mend the economy.
Sudan has faced soaring inflation since it separated with South Sudan a year ago -- taking with it more than 70% of Sudan's oil reserves. Sudan relied on its crude production for revenues and exports.
The nation's inflation has gone up by 30%, according to Sudanese Finance Minister Ali Mahmud.
Protesters vowed to hold more rallies Saturday when the ruling party celebrates 23 years in power. Organizers are calling for mass action using social media sites such as Twitter and Facebook, where they are posting pictures and details of upcoming protests.
In similar protests in January last year, students sought to replicate the Arab Spring that has swept over the Middle East. The government quashed those protests.