DAR ES SALAAM (Reuters) - Tanzania’s police said on Wednesday they were investigating the death of an opposition campaigner in the country’s commercial capital, while dissidents said the killing showed politically motivated attacks were continuing.

The East African country has been one of the continent’s most stable nations, but opposition leaders have recently expressed concern over a string of unexplained disappearances and violent attacks on political figures.

On Wednesday, police official Muliro Muliro told journalists that Daniel John, a campaigner for the main opposition party CHADEMA, was abducted on Monday and subsequently killed by unknown people in Dar es Salaam.

John had been involved in campaigning for a hotly contested parliamentary by-election in Dar es Salaam’s Kinondoni constituency due to be held on Saturday.

“The police force will not hesitate to take swift legal action against anyone or any group of persons that intends to sabotage the by-election,” Muliro said.

Ruling party officials were not immediately available for comment.

“Very sad that politically-motivated killings continue in our country. I condemn the killings in the strongest possible terms,” Zitto Kabwe, the leader of the opposition ACT-Wazalendo party said via Twitter.

In September opposition lawmaker Tundu Lissu, a fierce critic of President John Magufuli’s government, was shot several times by unknown gunmen in the administrative capital Dodoma. He left the country for treatment and has yet to return.

Magufuli’s government denies cracking down on the opposition. Authorities have said the cases remain under investigation.