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NAIROBI, Jan 3 (Reuters) - Kenyan police used teargas and water cannon against several hundred anti-government protesters chanting "Peace" as they sat down on a Nairobi highway on Thursday, blocking a major roundabout, a Reuters witness said.

"We are all Kenyans, there's no difference between us," shouted one protester.

About 100 riot police faced the crowd on a day when the opposition has called for a million-strong rally in central Nairobi against President Mwai Kibaki's disputed re-election.

The protesters, who had marched from the vast Kibera slum to about one mile (1.6 km) from Nairobi's city centre, wore white scarves, waved twigs and leaves -- symbols of peace -- and sang Kenya's national anthem.

"May we dwell in unity, peace and liberty. Let peace be found within our borders," they sang, in the anthem's words.

Some police looked on bewildered.

"We are fighting for our rights. That is why we have come to demonstrate in peace," said protester Tared Onyango, who is unemployed. (Reporting by Nicolo Gnecchi, Writing by Andrew Cawthorne)

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