Tanzanian police have said that two people were killed in the northern town of Arusha when security forces fired on protesters following the arrest of opposition leaders.
"Two people who suffered gunshot wounds died after being taken to hospital," Tobias Andengenye, police chief for the Arusha region, said on Thursday.
"Nine people are injured, including three police officers," he said.
Several thousand supporters of the Chadema party had gathered Wednesday in Arusha town centre to protest what they say is the fraudulent re-election of President Jakaya Kikwete at the end of October.
Several party leaders were taken in by police as they fired to disperse demonstrators trying to march on police headquarters.
The provincial police chief confirmed that 49 Chadema members were arrested, including its chairman Freeman Mbowe and its secretary general Wilbrod Slaa, an anti-graft campaigner who ran in the October presidential election and finished second.
Slaa, 62, is a lawmaker best known for having pioneered an anti-corruption drive that led to the resignation of prime minister Edward Lowassa, several legislators and the central bank governor.
In his speech to the crowd on Wednesday, Slaa called on police to release Mbowe and said Kikwete, the president, should stand down.
"They overstepped the authorisation we gave them," the police chief said, explaining that Chadema was only supposed to hold a meeting, not a march.
"They tried to invade police headquarters to free the people who had just been arrested," he added, saying that the files of the people arrested would be transmitted to the prosecutor's office.
On Thursday morning anti-riot police were patrolling the streets of Arusha, which is home to the International Criminal Tribunal for Rwanda and to the secretariat of the East African Community, an intergovernmental organisation for neighbouring countries.
Security forces had sealed off access to police headquarters but residents were going about their business normally.
Last month, police in the capital Dar es Salaam fired warning shots during a protest staged by opposition supporters seeking a new constitution, which Kikwete subsequently promised.
Political protests have been mounting in the east African country since incumbent Kikwete won October's general election, which were denounced as fraudulent by opposition groups.