A dispute over election results in Kenya that has pitted supporters of incumbent President Uhuru Kenyatta against his rival, Raila Odinga, intensified on Thursday, with the opposition presenting what it says is evidence of tampering with the electronic voting system.

The provisional results from Tuesday's vote gave Kenyatta a substantial lead over Odinga. However, opposition official Musalia Mudavadi claimed to have obtained "complete data" from election commission servers showing that Odinga was actually ahead by several hundred thousand votes. That claim echoed one made by Odinga shortly after the voting.

Mudavadi said the opposition had gotten the information from "confidential" sources inside the election commission. He said it showed "a serious attempt to try to either doctor or alter the final results."

Election commission Chairman Wafula Chebukati acknowledged that an attempt to hack the voting system was undertaken on polling day, but that it failed. The election commission has until Aug. 15 to report final results.

While the country was largely peaceful, it has a history of deadly violence following disputed elections. And there were areas of unrest as protesters in the capital, Nairobi, sparred with police, who fired tear gas.

NPR's Eyder Peralta, reporting from Kibera, said a protest there was of particular concern. "In 2007 after disputed elections, here in Kibera is the place that saw the most violence in the capital," he says. "Neighbors turned against neighbors, mostly along tribal lines, and I think we're seeing the beginning of that here."

"I haven't seen anyone get hurt, but there's palpable anger. There's at least one little shop that has been destroyed," Eyder said.

The U.S. State Department on Thursday urged candidates in Kenya to refrain from allowing their supporters to turn to violence.

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