Sudanese police fired tear gas at protesters in the capital Khartoum and its twin city Omdurman after midday prayers on Friday as organisers urged nationwide demonstrations over the next week against President Omar al-Bashir.Crowds chanting "freedom, peace, justice" demonstrated 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.Friday's protests came after organisers called for nationwide demonstrations over the next week demanding Bashir resign.The demonstrations that 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 Bashir's rule in three decades in power. "We will launch a week of uprising with demonstrations in every Sudanese town and village," the Sudanese Professionals' Association said.The group called for a major rally in Khartoum North on Sunday, to be followed by further demonstrations in the capital during the week.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook. The association, which has mobilised its membership to keep up the momentum of the protests, has also called for a rally later on Friday in the eastern town of Atbara, where the demonstrations first began.At least 22 people have been killed during the protests, including two security personnel, according to the authorities. Human rights groups have put the death toll much higher. Human Rights Watch said on Monday that at least 40 people had been killed, including children and medical staff.Analysts say the challenge now for organisers is to get protesters onto the street in numbers."Right now, some of the opposition groups and trade unions are trying to mobilise more protests, and probably they are thinking of how to escalate," said Matt Ward, senior Africa analyst at Oxford Analytica."But so far there hasn't been an escalation, they are persistent but they haven't risen in intensity in a significant way."Economic crisis Although the immediate trigger for the protests was the price of bread, Sudan has been facing a mounting economic crisis over the past year, led 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.Bashir and other officials have blamed Washington for Sudan's economic woes.Washington imposed a trade embargo on Khartoum in 1997 that was lifted only in October 2017. It restricted Sudan from conducting international business and financial transactions.The foreign currency shortages began with the 2011 secession of South Sudan, which took with it the bulk of oil revenues. But critics of Bashir say his government's mismanagement of key sectors and its huge spending on fighting ethnic minority rebellions in the western region of Darfur and in areas near the South Sudan border has been stoking economic trouble for years.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," Bashir said, brandishing his trademark cane.Across the Nile in Khartoum's twin city of Omdurman, hundreds of anti-government protesters blocked a main highway hours later, before riot police moved in. Three demonstrators were killed as police fired tear gas to disperse the protest, the authorities 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.The crackdown has drawn international criticism with Britain, Canada, Norway and the United States warning Khartoum that its actions would "have an impact" on its relations with their governments.Sudan has dismissed their concerns as "biased" and has insisted it is "committed to freedom of expression and peaceful demonstrations".