Tear gas was fired at thousands of supporters of Sudan's opposition Forces for Freedom and Change movement who had gathered in the capital Khartoum on Friday, witnesses said.
The source of the tear gas was unclear. The witnesses told Reuters there was no sign of police officers at the scene.
Mass protests erupted following a military coup in October. They have continued following a deal announced on Nov. 21 in which the military reinstated Prime Minister Abdalla Hamdok, who had been under house arrest.
The agreement between Hamdok and the military faces opposition from protesters who previously saw Hamdok as a symbol of resistance to military rule and denounced it as a betrayal.
U.S. Secretary of State Antony Blinken said on Twitter: "Many Sudanese people continue to demand respect for their basic human rights and to voice their enduring aspiration for a democratic Sudan. The U.S. continues to stand with them."
Khalid Omar Youssef, minister of cabinet affairs prior to the coup and prominent opposition figure who was arrested and released after the military took over, was on stage speaking to the crowds when the tear gas was fired.
Youssef then tweeted saying that "whether they fire tear gas or bullets on us, they will not silence us ... we will defeat the coup and our people will regain their freedom."
Other leading opposition figurers were scheduled to speak.
Live footage aired by Sudan's congress party following the tear gas firing showed seats haphazardly scattered.