War veterans have been at the centre of political violence
The body of Cain Nkala was found in a shallow grave on Tuesday, a week after he was kidnapped from his home by armed assailants. He had been strangled with his shoe laces.
Police have arrested six men, reportedly opposition members, and the minister of home affairs has been quoted as saying that the police are investigating leaders of the Movement for Democratic Change.
But the MDC leader, Morgan Tsvangirai, says this is a pretext to intimidate the opposition ahead of presidential elections due early next year.
More than 40 people have been killed in politically-motivated violence since President Robert Mugabe was defeated in a constitutional referendum in February 2000.
Stormed
Most of the violence has been blamed on supporters of Mr Mugabe, especially war veterans involved in the occupation of white-owned land.
Since Mr Nkala's abduction, there have been isolated cases of violence against MDC members in Bulawayo and Harare - both opposition strongholds.
On Saturday, war veterans, closely allied to President Robert Mugabe's Zanu-PF party, stormed the headquarters of the MDC in the capital.
"If they [the MDC] want a bloodbath, they will certainly get it," said Vice President Joseph Msika.
Prime suspect
Nkala was a prime suspect in the abduction of an MDC polling agent, Patrick Nabanyama, on the eve of the June 2000 parliamentary elections.
Mr Tsvangirai condemned the murder of Mr Nkala and called for the culprits to be brought to justice.
"Our conscience is clear," he said.
State television has shown two men confessing to the murder and says they are members of the MDC.
'Terrorism'
The MDC has not disputed that the men are members and secretary general Welshman Ncube has complained that they have not been given access to lawyers.
"This raises suspicions that the two young men were tortured into admitting something that they did not do," he said.
Home Affairs Minister John Nkomo says the murder was an act of "terrorism" perpetrated by "enemies of the government".
Mr Tsvangirai said that the authorities were practising double-standards by making such a fuss about the murder of Mr Nkala when "ignoring" attacks on the opposition.
Last year, Mr Mugabe issued an amnesty for political crimes committed in the run-up to last year's parliamentary elections - except for murder, rape and fraud.