- Sudan has been facing a mounting economic crisis over the past year, led by an acute shortage of foreign currency.
- 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.
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.
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," Matt Ward, senior Africa analyst at Oxford Analytica, said.
"But so far there hasn't been an escalation, they are persistent but they haven't risen in intensity in a significant way."