Nairobi, Kenya ? Clashes between rival tribes armed with machetes and bows and arrows on Friday marked the third, the bloodiest and what the government hopes is the last day of opposition protests over Kenya’s disputed presidential election.

With more than 20 people killed since Wednesday, the opposition announced a new strategy of economic boycotts and strikes to ratchet up pressure.

The U.S. ambassador, citing “many factors and underlying grievances,” compared Kenya’s violence to the 1968 race riots in the United States.

At a town hall meeting for Americans in Nairobi, Ambassador Michael Ranneberger said there was “a lot of cheating on both sides” in the Dec. 27 elections that pitted President Mwai Kibaki against opposition leader Raila Odinga.

Kibaki insists he won the election, but international and local observers say the vote count was rigged. Kibaki’s power has become more entrenched and he appears unlikely to accede to demands he step down. The opposition’s best hope may rest in wrangling a power-sharing agreement that might make Odinga prime minister or vice president.

The U.S. Embassy estimates that between 23,000 and 100,000 votes separated the two candidates. “You can’t have a recount and you can’t have a new election … so the two sides need to sit (together) and work things out,” Ranneberger said, suggesting the best solution was for the two to share power.