"McGee climbed onto the hood of the car while his aide snatched the keys from the ignition, then the diplomats used their mobile phone cameras to take photographs of the officer.

The confrontation was "a message to us, to try to control what we are doing," said U.S. Embassy spokesman Paul Engelstad, who was among around 50 diplomats and journalists on the tour.

Japanese, EU, Dutch and Tanzanian envoys and an Associated Press reporter were also in the convoy.

McGee noted the government has denied repeated reports from opposition officials and human rights groups of government-orchestrated violence against the opposition.

"The government has said, 'Present us with proof.' I think today we have done just that.

"We do have concrete proof of what is happening in the countryside and the government will have to listen," Mr McGee said."