The body of Chris Msando, charged with overseeing the country's crucial electronic voting system, was found naked with signs of torture in a forest on the outskirts of Nairobi over the weekend.
"This is a source of very, very significant concern and we want to tell the IEBC (election commission) that there are a lot of people out there who are concerned about the killing," said George Kegoro, director of the Kenya Human Rights Commission which led the march of several dozen people to IEBC headquarters.
"Kenya has an unfortunate history of assassinations for political expediency and there is a chilling familiarity in the sequence of events... that makes it difficult to dismiss the sinking feeling that we have been here before," he said.
Msando, who had been declared missing on Saturday, was identified by family members on Monday, prompting an outpouring of condemnation from rights groups and the international community.
In a joint statement, the ambassadors of the United States and Britain said they were "gravely concerned" by the murder and offered their countries' assistance in the probe.
Msando had made several television appearances in the days before his death to assure Kenyans of the reliability of the electronic voting system which crashed during the 2013 elections leading to accusations of poll fraud.
He had been due to lead a test run of the system on Monday, which was postponed.
The electoral body's Chairman Wafula Chebukati called on government to provide security to IEBC staff, but sought to play down fears Msando's killers had obtained any sensitive information.
"Whoever tortured him I don't think he got anything. All our passwords are intact. We have service providers and, as it is, none of the commission employees have these passwords so let us not speculate," Chebukati said.
Tension has been rising in the lead up to the election, in which incumbent Uhuru Kenyatta will face off with rival Raila Odinga in the presidential race. Opinion polls place the two men in a dead heat.
Odinga has insisted that Kenyatta plans to rig the election while the incumbent accuses him of seeking to stoke violence.
The country is still haunted by the memory of post-election violence which broke out in 2007 after a disputed poll, leaving more than 1,100 dead.
"There's plenty reason to believe that this cruel and selfish act is related to the election," said anti-corruption campaigner John Githongo.
Kenyatta called on the nation to remain united ahead of the election and ensure that "the ideal for which he lived -- the free and fair choice of our leaders -- is realised."
