Thousands of protesters chanting "revolution is the people's choice" took to the streets on Tuesday in a city southeast of the Sudanese capital to call on longtime leader Omar al-Bashir to step down, the latest in nearly three weeks of anti-government demonstrations.The demonstration in Gadaref came just hours after Sudan's Interior Minister, Ahmed Bilal Othman, told Parliament that police have arrested 816 people since protests erupted Dec. 19, initially over price rises and shortages but which soon shifted to calls for al-Bashir to step down.Also on Tuesday, Human Rights Watch said at least 40 people have been killed in the protests and accused security forces of using live ammunition and excessive force against the protesters as well as arbitrary detentions. Othman, the minister, said 19 people have been killed in the protests, including two members of the security forces. Al-Bashir has ordered an investigation into the killings.Authorities also have arrested scores of opposition leaders and slapped nighttime curfews and emergency laws in a string of cities across Sudan. They also suspended classes in schools and university across most of Sudan.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook. The protesters in Gadaref wanted to reach the provincial legislature to deliver a note demanding that al-Bashir, in power since he led a military coup in 1989, step down. Police used tear gas and fired in the air to disperse them. There were no reports of casualties Tuesday.Of the reported 40 people killed in the protests, activists say 23 of them have fallen in Gadaref.There have been calls for a march to the national legislature in Khartoum's twin city of Omdurman on Wednesday, also to deliver a note demanding that al-Bashir quits. Political parties allied with al-Bashir's National Congress Party are planning a rival demonstration Wednesday on the other side of the city near the airport.The opposition demonstration in Omdurman could potentially attract thousands given the city's track history of dissent and because it is a stronghold of the al-Ansar, descendants of the followers of Imam al-Mahdi, a messianic religious figure from the late 1800s. Al-Ansar's political wing is the large Umma party, whose headquarters is also in Omdurman.Sudan's Parliament is packed with al-Bashir's loyalists, who are campaigning to amend the constitution to allow the general-turned-president, already one of the longest serving leaders in the region, to run for a new term in 2020 elections.