At least three people were killed and several injured on Saturday evening by an explosion at an election rally held by Tanzanian opposition party Chadema, witnesses and the state broadcaster said.
Police said they were still investigating the cause of the blast in the northern city of Arusha but senior political figures made clear they believed the incident was a bombing.
"At least two children and one woman died in the explosion. More than a dozen people were injured and rushed to hospital ... it was a nightmare," said Rose Shayo, an eyewitness.
The blast occurred near the main stage as party chairman Freeman Mbowe was addressing supporters in the Soweto area of Arusha, the Tanzanian Broadcasting Corporation reported.
"We have launched an investigation into the incident... there was a lot of panic and chaos after the explosion went off," Advera Senso, national police spokeswoman, told Reuters.
Chadema officials have long complained against a government crackdown on opposition demonstrations and public rallies. They were not immediately available for a comment after the explosion in Arusha.
"I'm sincerely saddened by a bomb blast in a campaign rally in Arusha. My condolences to those who lost their loved ones," ruling CCM party member of parliament Hamisi Kigwangalla said in a statement.
"I join you in condemning (those responsible) and consoling those who lost their loved ones as we await an official government statement," Foreign Minister Bernard Membe said on his Twitter account.