Nairobi - Kenyan opposition leader Raila Odinga called on Sunday for "international intervention" in the country's election crisis, saying at least 31 supporters have been killed by police and militia since his return from an overseas trip on Friday.Odinga said Kenya "was being pushed to the precipice" after residents of a Nairobi slum protested the killing of at least 13 people in an overnight attack by unknown gunmen. "This is state-sponsored thuggery," Odinga said. An area lawmaker was shot in the leg during protests that followed the killings, Odinga said.Police said four people were killed in the attack, but mortuary attendants told The Associated Press that 13 bodies, all with bullet wounds, had been brought in.Odinga said 18 people were shot dead on Friday when police tried to stop his supporters from lining the roads to welcome him after speaking engagements in the US and Britain. Mortuary records corroborate Odinga's death toll. Police said five people were killed by mobs.Tensions have risen since the Supreme Court nullified President Uhuru Kenyatta's re-election in August, citing irregularities, after Odinga challenged the results. The court ordered a fresh vote last month which Odinga boycotted, citing a lack of electoral reforms.Kenyatta won the repeat election in October, but his victory is again being challenged at the Supreme Court by civil society activists and a politician. The court will announce its decision on Monday.There have been concerns about intimidation of the justices, who failed to muster a quorum to decide on a last-minute petition that sought to postpone last month's election. One justice's bodyguard was shot and seriously wounded hours before the expected judgment.Odinga has rejected Kenyatta's victory.The nullification of Kenya's August election was the first time a court in Africa had overturned a presidential vote. With this weekend's death toll nearly 100 people have died in political unrest since then, the majority opposition demonstrators shot by police during protests.