The Chinese government is reported to have sent thousands of soldiers and police to quell unrest in the central province of Hunan.
Up to 10,000 people took to the streets in Jishou to demand money back from an allegedly fraudulent fundraising firm, a Hong Kong-based rights group said.
In another protest in the eastern port of Ningbo, 10,000 workers clashed with police, the group added.
Social unrest is common in China, but rarely on this scale.
The Hong Kong-based Information Centre for Human Rights and Democracy said that, in both protests, violent clashes erupted between angry crowds and local authorities.
In Jishou 50 people were injured in rioting, and police arrested 20 people, the group said.
According to Xinhua news agency, the protesters blocked roads and trains to demand that the government take action after a fundraising company "failed to pay them back as promised".
The Jishou government admitted in a statement that armed police were drafted in to disperse the crowds, but did not mention if anyone was hurt.
In the second incident, thousands of migrant workers confronted police in Ningbo to protest about the injury of a man in a local factory.
The protests are the latest in a series of confrontations over social issues in China - many of which stem from grievances over alleged corruption and local authorities' abuse of power.
In June, thousands of people rioted in Guizhou province over claims that police had covered up the rape and murder of a girl.