The allegations could cloud the new coalition government
Kenya's political parties spent millions of dollars bribing voters in last year's elections, a survey says.
The Coalition for Accountable Party Finance says out of $90m raised by the parties, 40% was used as bribes.
The report also says public corporations contributed to President Mwai Kibaki’s campaign via his party.
Some 1,500 people died and 600,000 others were displaced during violence following the disputed poll, before the rivals agreed to share power.
The allegations relate to both of Kenya's main parties - the Orange Democratic Movement (ODM) and Mr Kibaki's Party of National Unity (PNU).
ODM leader Raila Odinga last week became Kenya's prime minister.
The disputed elections were the most competitive in Kenyan history.
The lobby group now wants legislation to compel politicians to reveal their sources of income and expenditure during election campaigns.
Charles Otieno, who heads the Coalition for Accountable Party Finance, says new laws are needed urgently.
"Most of the money politicians use is from tax-payers and without such laws they will continue to spend massive amounts from the public coffers with impunity," Mr Otieno said.
The report alleged that the state-owned electricity company charged customers around $8m too much and then donated the surplus to a political party.
But the Kenya Power and Lighting Company (KPLC) denies the allegations.
"These accusations are completely untrue and the organisation should come up with evidence to prove their claims," KPLC’s chief executive Don Priestman said.
But Mr Otieno insists several government departments created inflated invoices and used the money for campaigning.
The report says the parties also raised funds from Kenyans in the diaspora and Kenyan companies based overseas.
The lobby group says candidates should declare their wealth before they contest an election, to prevent such abuses of power.