NASA leader Raila Odinga said on Tuesday he wants an interim government to run the country for six months while the constitution is reviewed to curb President Uhuru Kenyatta's authority.
He told Reuters in an interview that he was open to talks with Uhuru over a constitutional review aimed at lowering the risk of violence from minority groups who feel excluded from power.
Uhuru was re-elected with 98 percent of the vote on October 26 after Raila boycotted the election, saying it would not be fair.
The repeat presidential vote followed an election in August, which Uhuru also won but which was nullified by the Supreme Court on procedural grounds.
Raila's supporters have repeatedly protested against the results, and at least 51 people have been killed in the turmoil since August.
This has generated an atmosphere of uncertainty in East Africa's richest economy.
"A pure presidential system enhances ethnicity because each community believes that they are not safe unless their man is at the top," Raila said in the interview in his office.
Three of Kenya's presidents since independence from Britain in 1963 have been from the Kikuyu ethnic group and one has been from the Kalenjin group, although there are 44 recognised ethnic groups in the nation.
The current president is a Kikuyu and his deputy is a Kalenjin, and many of Raila's supporters, who are largely drawn from other ethnic groups, say they feel excluded from power.
The 2010 constitution devolved some power and money to Kenya's 47 counties, spreading the opportunities for power and political patronage. But most of the budget and the power is still in the hands of the central government, something Raila wants to change.
"We had a new constitution that we enacted in 2010; we think that now it is time to revisit it," he said.
A constitutional review could strengthen institutions like the election board and cut down the powers of the presidency, he said.
"We think that maybe six months will be required to carry out all these changes that we need in this country."
Raila served as prime minister in a coalition government that was crafted in 2008 after two months of violence following a disputed election killed 1,200 people.
After the October 26 election, Raila's opposition alliance called for civil disobedience, including protests and a boycott of products and services by firms friendly to Uhuru's government, to force reforms.
The opposition did not challenge Uhuru's October 26 victory at the Supreme Court and there has been no protest action, but voter and rights activists lodged cases on Monday.
The court has 14 days to review them.