Human Rights Watch has urged Sudan's government to show restraint and not use lethal force against protesters during the weeks-long demonstrations against President Omar al-Bashir.
The New York-based group, citing estimates from independent groups, said on Monday that Sudanese forces have been using tear gas and live ammunition against protesters, killing more than a dozen people.
The statement came ahead of renewed protests on Monday, with demonstrators in the capital, Khartoum, expected to try to march on Bashir's palace to demand he step down.
Bashir, who has been in power since 1989, vowed in a meeting with police commanders on Sunday that his government would not tolerate any attempt to undermine the stability and security of Sudan, according to the state news agency.
He also sought to justify the killing of protesters, quoting from Islam's holy book, the Quran, according to a video clip of his comments.
"It's deterrence to others so that we can maintain security, which is a valuable commodity and, God willing, we will not risk the security of the citizens or the nation," said Bashir.
Jehanne Henry, associate director at Human Rights Watch's Africa division, said Bashir's speech appeared to "justify excessive use of force instead of condemning this brutality".
"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," he added.
Amnesty International has said it has "reliable reports" that 37 protesters were killed in the first five days of protests, which began December 19.
The government has acknowledged 19 deaths.
The protests were sparked by an increase in bread prices, with demonstrators rallying against the government tripling the price of a loaf of bread from one Sudanese pound to three ($0.02 to $0.06).
Since then, demands have widened to include calls for Bashir's resignation.