WASHINGTON, May 14 (Reuters) - The United States protested to Zimbabwe's government on Wednesday over the "harassment" of the U.S. ambassador and other diplomats there and criticized Harare's lengthy buildup to a runoff presidential election.

Washington issued a formal protest note to Harare about the treatment of diplomats who were questioned by police on Tuesday as they returned from visiting victims of post-election violence, State Department spokesman Tom Casey said.

"Certainly it's inappropriate behavior and we want to make sure the government is aware of our concerns about this issue," Casey told reporters.

Casey also called for Zimbabwe to set a date for the runoff presidential election. Harare's government announced earlier that the runoff had been delayed and will now be held within 90 days of May 2, when the official results of a disputed presidential vote were released.

Results from the Zimbabwe Electoral Commission of the disputed March 29 election showed opposition leader Morgan Tsvangirai beat veteran President Robert Mugabe in the presidential poll but not by enough to avoid a run-off.

"There's a whole list of things the Zimbabwean government can and should be able to do if it is serious about holding a runoff that is free and fair," Casey said.

"The delay without a date being set just continues to underscore the fact that the government is not taking the steps it needs to take," he said.

In addition to setting a runoff date, Zimbabwe should allow independent supervision of the country's election commission, allow observers throughout the country from the regional Southern African Development Community and let the opposition print campaign materials without being thrown in jail, he said.

The United States has had harsh words for Zimbabwe's government, accusing it of harassing and attacking the opposition following the disputed March 29 poll.

Casey said if a runoff were to happen, there was an obligation on the part of all the international community to push the Zimbabwe government to make sure certain conditions were in place.

(Editing by Cynthia Osterman)

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