- Last month senior Chadema leaders, Mr Mbowe and his deputy Tundu Lissu, were arrested after they attempted to hold a youth rally.
- Police banned the rally, saying it was intended to cause violence.
Tanzania's President Samia Suluhu Hassan has denounced the brutal killing of a senior member of the main opposition party Chadema, who was kidnapped, beaten and doused with acid.
On Friday, Mohamed Ali Kibao, 69, was forced off a bus by suspected security agents while travelling from the country's biggest city, Dar es Salaam, to his hometown Tanga. His body was found in Ununio, the waterfront district of Dar es Salaam, local media reported.
The post-mortem found that Mr Kibao had been “severely beaten and had acid poured on his face”, party chairman Freeman Mbowe told the AFP news agency.
President Samia condemned the “brutal acts” and called for an investigation into the murder.
“I have ordered the investigation agencies to bring me detailed information about this terrible incident and others like this as soon as possible,” she said in a post on X, formerly Twitter.
“Our country is a democracy, and every citizen has the right to live,” she added.
The United States Embassy in Tanzania voiced their support for an “independent, transparent, and prompt investigation” into the killing of Mr Kibao.
Mr Kibao's killing comes amid concern by the opposition and rights groups of a clampdown on political activity.
“We cannot allow our people to continue disappearing or being killed like this. The lives of Chadema leaders are currently at risk,” Mr Mbowe told AFP.
Mr Kibao was a retired military intelligence officer and joined Chadema in 2008. He will be buried on Monday in the Darigube district of Tanga city.
The killing of Mr Kibao has sparked widespread condemnation across Tanzania, with many asking the government to take action over reports of several other people being abducted and killed.
The US Embassy in Tanzania said: "Murder and disappearances, as well as last month’s detentions, beatings, efforts to disenfranchise citizens ahead of elections, should have no place in a democracy".
Last month senior Chadema leaders, Mr Mbowe and his deputy Tundu Lissu, were arrested after they attempted to hold a youth rally.
Police banned the rally, saying it was intended to cause violence.
In July, an artist was accused of burning an image of President Samia and sentenced to two years in prison.
Many people fear Tanzania could be returning to the repressive rule of late President John Magufuli, despite his successor Ms Samia lifting a ban on opposition gatherings and promising to restore competitive politics.
In August, Human Rights Watch said the increase in arrests of opposition activists was a "bad sign" with the 2025 presidential elections around the corner.