Sudanese security forces have fired tear gas to disperse hundreds of protesters in the capital Khartoum and its twin city Omdurman as organisers urge nationwide demonstrations over the next week calling for the resignation of President Omar al-Bashir.

Crowds chanting "Freedom, Peace, Justice" demonstrated on Friday in two areas of Khartoum and in Omdurman just across the Nile, witnesses said.

They were quickly confronted by volleys of tear gas from riot police.

The demonstrations, which first erupted on December 19 over a government decision to triple the price of bread, have swiftly escalated into broader protests that are widely seen as the biggest threat to al-Bashir's three-decade rule.

Friday's protests came after organisers called for nationwide demonstrations over the next week demanding that al-Bashir resign.

"We will launch a week of uprising with demonstrations in every Sudanese town and village," the Sudanese Professionals' Association said.

More protests in #Omdurman, one day after Omar Al Bashir's security forces killed 3 & fired tear gas to disperse dozens of demonstrators on Thursday in the latest of several weeks of #Sudan 🇸🇩 anti-gov protests sparked by economic and political grievances.https://t.co/AbuKFpBR4r pic.twitter.com/lnABkcqMEA — SaadAbedine (@SaadAbedine) January 11, 2019

At least 22 people have been killed since the protests began, according to the authorities.

Human Rights Watch, however, said on Monday that at least 40 people had been killed, including children and medical staff.

Reporting from Khartoum, Al Jazeera's Hiba Morgan said that while al-Bashir has remained defiant regarding the protests against him, so have the protesters.

"They say they will continue to come out into the streets, they will continue to make their voices heard, and they will continue to demand that he step down, something that [al-Bashir] said he's not going to do until elections in 2020," Morgan said.

"Even then, his party says they are going to try to re-elect him so that he can be nominated for presidency next year. So it's not clear if these protests will ever end.

"People are saying that they're fed up with his 29-year rule and that his promises to try to reform the economy, to try to improve the living situation for people is something that they've heard over and over again and only him stepping down would be the solution."

Economic crisis

Although the immediate trigger for the protests was the increase in the price of bread, Sudan has been facing a mounting economic crisis over the past year, partially caused by an acute shortage of foreign currency.

Repeated shortages of food and fuel have been reported in several cities, including the capital Khartoum, while the cost of food and medicine has more than doubled.

Al-Bashir and other officials have blamed Washington for Sudan's economic woes, having imposed a trade embargo on Khartoum in 1997 that was lifted only in October 2017.

The president has remained defiant, telling thousands of loyalists at a Khartoum rally on Wednesday that his government would not give in to economic pressure.

"Those who tried to destroy Sudan... put conditions on us to solve our problems, I tell them that our dignity is more than the price of dollars," al-Bashir said.

Across the Nile in Khartoum's twin city of Omdurman, three demonstrators were killed on Wednesday as police fired tear gas to disperse the protest, the authorities said.

Morgan said that there are concerns among the protesters that the government will escalate its use of force in an attempt to put down the protests.

"People are saying that the force used against them by the government is brutal and that it increases day by day," Morgan said.

Human rights groups say more than 1,000 people have been arrested since the protests began, including opposition leaders, activists and journalists as well as demonstrators.