Police in Sudan have fired tear gas at anti-government protesters in the eastern city of Kassala, according to eyewitnesses, as demonstrations against President Omar al-Bashir's three-decade rule entered its fourth week.

Wednesday was the first time protesters took to the streets in Kassala, located near Sudan's border with Eritrea, since massive anti-government demonstrations started in December in which at least 24 people have died.

Eyewitnesses told AFP news agency that protesters shouting "freedom, justice and peace" were confronted by police officers who fired tear gas at them.

On January 7, Kassala witnesed the first pro-government rally held by supporters of al-Bashir, which was followed days later by another in the capital, Khartoum.

Since December 19, Sudan has seen several large-scale protests in most major cities and towns.

Demonstrations initially erupted in the northeastern town of Atbara after the government raised the price of bread, but then quickly flared to other regions before spreading to Khartoum.

March on the presidential palace

More than 800 people have been arrested since the unrest began, said officials, insisting that the situation has now stabilised even as protests continue.

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Protest organisers have called for a march on the presidential palace in Khartoum on Thursday, along with simultaneous demonstrations in several cities, including Port Sudan, Gadaref and Madani.

They are demanding the removal of al-Bashir, the 74-year-old general-turned-politician, who has ruled the north African country since 1989.

An embattled al-Bashir has promised economic development but has refused to step down. Instead, he has called on opposition leaders to wait until the elections scheduled to be held in 2020.

Sudan's ruling party has pressed ahead with plans to change the constitution, so Bashir can stay in office beyond his present term, which ends in 2020.