Gunmen killed a Mozambique opposition lawyer and a party official after firing multiple rounds at a car in which they were travelling on Saturday, rights groups said, ratcheting up tensions ahead of protests against a disputed election result.
New opposition party Podemos and its presidential aspirant Venancio Mondlane reject provisional results showing a likely win for Frelimo - the party that has ruled Mozambique for half a century - and its candidate Daniel Chapo.
They have called for a nationwide strike on Monday.
Mozambican civil society election observer group More Integrity said the attack happened in the Bairro Da Coop neighbourhood of the capital Maputo, killing Podemos lawyer Elvino Dias and party representative Paulo Guambe.
Human Rights Watch and Mozambique's Center for Democracy and Human Rights (CDD) also issued statements confirming the attack.
"They were brutally assassinated (in a) cold-blooded murder," Adriano Nuvunga, CDD director, told Reuters by telephone.
"The indications that around 10 to 15 bullets were shot, and they died instantly," he added, describing it as a "message" to opposition protesters planning to convene on Monday.
A Frelimo spokesperson did not respond to requests for comment.
The European Union and Mozambique's former colonial ruler Portugal condemned the killing and called for an investigation.
Mondlane's rise to become Mozambique's main challenger was a threat to Frelimo, but also to former official opposition party Renamo - once a rebel outfit backed by racist white regimes in South Africa and Rhodesia (Zimbabwe) during the Cold War.
Alex Vines, Africa programme director at London-based international affairs think tank Chatham House, called the killing a "serious escalation" that raised tensions ahead of Monday's strike.
Western observers have cast doubt on the credibility of the poll, noting reports of vote buying, intimidation, inflated voter rolls and poor transparency in collation - problems that have marred most polls since Frelimo introduced democracy in 1994 after two decades in power.
Full results are expected on Oct. 24, but many fear Monday's protest could turn bloody. Mozambique's security forces have opened fire on protesters, including after last year's local elections, rights groups say.
