CAIRO -- Sudanese police on Monday used tear gas to disperse thousands of protesters shortly after they began to march toward the presidential palace in Khartoum to demand that President Omar al-Bashir step down, according to activists and video postings.

Some activists said police used live ammunition. Video clips posted online showed pools of blood outside a small eatery in the city center. Another showed protesters carrying a man whose head and shirt were bloodied. Scores of demonstrators run away amid the sounds of gunfire and screaming.

A female protester's voice is heard urging others, "don't run," as those around her violently cough from the tear gas. "I am going to die," says another woman, apparently overwhelmed by the gas.

Activists said scores of protesters have been detained and at least four suffered gunshot wounds. They spoke of at least three fatalities and scores of protesters wounded, but that could not be independently confirmed. Sudanese authorities heavily restrict media coverage of demonstrations.

Thousands have taken to the streets in Khartoum and other cities in protests over the past two weeks. An umbrella of independent professional unions called for the march to the palace after a similar march last Tuesday was broken up by police.

Protesters gathered in several locations in Khartoum and its suburbs, scattering when police fired tear gas and then regrouping again. The protests lasted for at least four hours.

Hundreds of security forces were deployed in Khartoum ahead of the protests, with some of them forming a security ring around the presidential palace. Snipers in civilian clothes could be seen on rooftops in central Khartoum.

In one video clip, scores of protesters in a Khartoum neighborhood pointed to a sniper on the roof of a six-story building and shouted "Sniper, we can see you!"

Before clashes erupted, the demonstrators in central Khartoum sang the national anthem. They chanted "Peaceful!" and "Oh, Sudan, we sacrifice our lives and blood for you."

They also chanted, "The people want to bring down the regime," which was the main slogan of the Arab Spring revolts of 2011.

Earlier Monday, Human Rights Watch urged al-Bashir's government to instruct security forces not to use lethal force against protesters, citing independent groups as saying 40 people have been killed since the protests erupted nearly two weeks ago.

The New York-based group said Sudanese forces have been using tear gas and live ammunition against protesters, as well as beating and arresting many. Scores have been injured and hundreds arrested, it said. Last week, the United States, Britain, Norway and Canada said in a joint statement that they have "reliable reports" that Sudan's security forces were using live fire.

Al-Bashir, an autocratic leader who came to power in a 1989 military coup, vowed in a meeting with police commanders Sunday that he would not tolerate any attempt to undermine stability and security, according to the state news agency. An Islamist, he also sought to justify the killing of protesters by quoting from the Quran, according to a video clip of his comments.

"The objective is not to kill the protesters, but ... to safeguard the security and stability of citizens," he said.

"President al-Bashir appears to be making public speeches that justify excessive use of force instead of condemning this brutality," said HRW's Jehanne Henry. "With more protests planned, Sudanese authorities should send an unambiguous message to all security forces to respect the rights of protesters and not to use lethal force."

Amnesty International has said it has "reliable reports" that 37 protesters were killed in the first five days of protests, which began on Dec. 19. The government has acknowledged 19 deaths.

Monday's attempted march on al-Bashir's palace is the second such attempt — thousands tried to reach the white neo-colonial building in central Khartoum last Tuesday, clashing with policemen who used tear gas and batons to disperse them.

Although the protesters never reached the palace, their action showed the depth of popular discontent with al-Bashir's rule. Protesters numbering in the hundreds or very low thousands gathered in a dozen or so venues across the city Tuesday and fought pitched battles with police for hours before they dispersed after nightfall.

This time, the unions are urging protesters to stay on the streets until they usher in 2019 so that they can mark the anniversary of Sudan's independence on Jan. 1, 1956.

Sudan's economy has stagnated for most of al-Bashir's rule. He has also failed to unite or keep the peace in the religiously and ethnically diverse nation, losing three quarters of Sudan's oil wealth when the mainly animist and Christian south seceded in 2011 following a referendum.

A year earlier, al-Bashir, now in his mid-70s, was indicted by the International Criminal Court for genocide in Sudan's western region of Darfur.