By Adam Mynott
BBC World Affairs correspondent
The two men were shot outside the University of Nairobi
Killings do not come more cold-blooded and calculated.
Oscar Kamau Kingara and John Paul Oulu were shot at close range while their car was standing in traffic during Nairobi's rush hour last night.
The men were killed in State House Road just yards from the heavily guarded residence of Kenya's President Mwai Kibaki. Two men riddled the vehicle with automatic gunfire and then walked away.
Hours earlier, during his weekly media briefing, Dr Alfred Mutua, the Kenyan government spokesman, accused the human rights organisation headed by Mr Kingara of being a front and a fund-raising body for a banned criminal sect called the Mungiki.
The Oscar Foundation, named after its now-dead founder, has been at the forefront of protests about alleged extra-judicial killings by police.
Mr Kingara had given the UN evidence of alleged police abuses
Kenya's security forces have spent the past two years trying to crack down very heavily on Mungiki followers after the sect carried out a series of brutal killings.
Earlier in the day there had been a big protest in Nairobi about police behaviour and claims that hundreds of young Kenyans had been killed by police and other security forces.
The protest followed the publication earlier in the week of a UN report by Professor Philip Alston into police operations in Kenya. It was, by UN standards, highly critical, accusing the police of being a law unto themselves and of killing with impunity.
Professor Alston called for the sacking of Kenya's police chief, Hussein Ali, and the resignation of the Attorney-General, Amos Wako.
No surprise registered when the Kenyan government rejected the report (which they had commissioned) and accused Professor Alston of exceeding his brief, which was to draw up an independent assessment of alleged illegal killings by police.
The Mungiki is a murderous organisation that traces its origins to the Mau Mau rebellion against British colonial rule in the 1950s and '60s.
Nairobi has seen lethal clashes between police and Mungiki mobs
It gained prominence in the 1980s when it coalesced around attempts to protect land belonging to Kikuyu farmers in the Kenyan highlands north of Nairobi.
It has since spread its tentacles into many other areas of Kenyan life and it turned criminal.
It has raised a lot of money by extorting money from Kenya's ubiquitous matatu buses.
Virtually every matatu operating in Central Province in Kenya has had to pay a "levy" of a few hundred Kenyan shillings (less than $10) a day to operate.
Mungiki has also recruited among the country's large population of disaffected Kikuyu youth. It has been used by some Kenyan politicians to intimidate the electorate and frighten off political opposition.
Mungiki mobs surfaced during Kenya's troubled election in December 2007. More than 1,500 people were killed in an orgy of violence which followed the disputed result.
Warring groups divided down ethnic lines, and while I was covering the violence I saw gangs wearing tell-tale Mungiki emblems (red scarves and bits of scarlet cloth tied around forearms) on the rampage.
Police behaviour, Mungiki violence and now the murder of two human rights activists call into question the rule of law in Kenya. Kenyan society is trying to heal itself after the inter-tribal violence.
It has a coalition government intended to draw together both political and ethnic differences. But the new joint administration is riven by divisions and gives the appearance of being deeply corrupt.
The murder, rape and rioting which threatened to plunge Kenya into civil war a year ago have ended, but Kenya still faces a crisis of huge proportions.