District of Columbia procurement rules were violated in the purchase of a $647,700 punch-card voting system chosen by the city's Board of Elections and Ethics, city auditor Matthew S. Watson contended yesterday.

In a report to the mayor and the City Council, Watson also charged that the elections board "unlawfully attempted to frustrate our investigation by refusing to give access" to its files or to the equipment that was purchased.

James L. Denson, elections board chairman, denied both accusations and said the board "strongly takes exception to the innuendoes of impropriety which appear throughout" Watson's report.

Watson's report dealt with the board's decision in July to buy the Datavote system, used experimentally in two recent city elections, from the Diamond International Corp. Watson said the city's General Services Department, the municipal purchasing agency, did not independently review details of the contract.

Only six days after the voting devices and booths were received -- to be used in the Nov. 6 school board election -- the board expedited payment to the company, Watson said. Such an early payment was "extraordinary," and cost the city $5,000 in interest it could have earned if it waited to pay the normal 30 days after billing.

A rival manufacturer of voting devices, Computer Election Systems, Inc., has filed suit in D.C. Superior Court against the Datavote contract, and has protested it to the U.S. General Accounting Office.

Denson, in a letter responding to a draft of Watson's report, defended the board's purchase decision.

He said the documents sought by Watson were not available in the board's office because they had been given to city lawyers for use in defending the lawsuit challenging the purchase.

Denson said the prompt payment was made by the city's accounting office because the board, obeying a mayoral order, sent the bill promptly to that office.