The police force in Kenya remains the most corrupt government department for a sixth year running, a report by Transparency International (TI) says.
President Kibaki promised to fight corruption when elected
Richard Leakey, who heads TI's Kenya branch, said despite reforms in the force, policemen still demand bribes.
The report says Kenyans still encountered bribery in more than half of their interactions with government and private institutions.
Kenyan President Mwai Kibaki promised to fight graft when elected in 2002.
Kenya ranks 142nd on TI's 163 nation global corruption list, only above Guinea, Sudan, Equatorial Guinea and Ivory Coast in Africa.
"It is very difficult to know why Kenya is still grappling with corruption; I think the political will to exercise zero tolerance is not that developed," Mr Leakey told BBC Focus on Africa programme.
The report, which reviews the bribery index in 2006, also named lawyers and managers of the recently launched constituency development funds as the other group that ranks high in soliciting for bribes.
TI says the average number of bribes more than doubled but the average size of the bribe fell to $18.35 (£9) from $24 (£12) within the period in review.
The survey also showed a marked increase in bribery relating to employment, with job seekers paying employers bribes to secure work.
Kenyans go to the polls in December and observers say President Kibaki, who is seeking a second term in office, may be judged harshly by voters disappointed by his government's failure to tackle graft.