Kenya's badly paid police force is the most bribe-prone institution in the country, according to a report issued by an anti-corruption watchdog on Tuesday.

Corruption has grown worse in Kenya since President Mwai Kibaki swept to power in 2002 vowing to stamp out the graft that was deterring investment in east Africa's biggest economy.

Kenya ranks 142nd on Transparency International's (TI's) 163-nation global corruption list -- above only Guinea, Sudan, Equatorial Guinea and Ivory Coast in Africa.

The new Kenya Bribery Index, compiled by TI's local branch, found that Kenyans encountered bribery in 54% of interactions with officials last year versus 47 percent in 2005.

The average number of bribes more than doubled to 2,5 from 1,2 per person, but the average size of the bribe fell to 1 236 shillings from 1 700 shillings.

Shortly after the landmark 2002 elections, signs advising Kenyans to report corruption were posted in almost every public office.

There were even instances of Kenyans making a citizen's arrest on police officers who asked for "kitu kidogo" -- "something small" in the local Swahili language.

But the initial euphoria wore off and many Kenyans say small-scale corruption remains a feature of daily life.

Although the majority of bribes in 2006 were paid to the police, its score improved to 46,6 from 60,3, partly because the government's decision to scrap road licences removed one reason for officers to stop cars in search of a bribable offence.

The study also showed the gap in bribes between the police and the second worst institution, the Transport Licensing Board, had narrowed, while the number of areas in which the police ranked worst fell to two out of six from four out of six.

"Reform in the force seems to be bearing fruit. However, media reports show that police are more blatant in demanding bribes and they still rank the worst in terms of likelihood and frequency," said TI's Kenya steering committee chairperson Richard Leakey.

"We cannot start celebrating yet," he added.

The study revealed a marked increase in bribery relating to employment, with job seekers paying employers 42% more in bribes to secure work.

Critics have accused the government, which stands for re-election this year, of failing to meet its promise of creating 500 000 jobs a year.

In third place on the index were public colleges and universities, in fourth place the Immigration Department and fifth place the Ministry of Local Government.

The index saw the entry of 14 new institutions -- including the Constituency Development Fund Office, which hands out money for services to Kenya's 210 constituencies, in eighth place.