The United States has blocked an Inter-American Development Bank loan to Guyana for a key rice-growing project, but U.S. officials denied charges that the action was prompted by political opposition to the country's leftist government.

Guyana is seeking two loans totaling $37.8 million to finance the second stage of the project--construction of secondary canals and drainage systems that Guyanese officials hope will increase rice exports and raise the standard of living in the South American country.

At the meeting yesterday of the bank's 12 executive directors, the United States cast the only vote against the first loan for $10 million, from what is known as the bank's "soft-loan window."

The United States has veto power over the soft-loan window because these loans given on easy terms must be approved by two-thirds of the voting power of the bank's 43 member countries, represented by the 12 executive directors. The United States owns 34 percent of the bank's shares and therefore can block a two-thirds majority.

Two weeks ago, Guyana charged that the United States intended to block the loan for political reasons. Yesterday, a spokesman for the Guyanese government, John Gelinas, reiterated the charges and warned that a delay in the second phase would threaten the project.

Treasury Department officials, who set U.S. policy at the bank, yesterday denied that politics was involved. The United States voted against the loan because it believed Guyana's rice-growing programs had too many government pricing regulations thattive to work, Treasury officials said.

In the past, Guyana, a former British colony, has criticized U.S. political and economic policies.

After the vote, Joseph Tyndall, the alternate for the executive director for the Caribbean, requested that the second loan request and a $1 million grant for technical assistance--which also requires two-thirds approval--be withdrawn.

The total cost for the second he project is $49 million--$37.8 million in loans and $11.2 million in local contribution. The first stage of the project--construction of the primary canals--cost $49.5 million and was financed totally through soft loans, bank officials said. It is expected to be completed within the next two weeks.

In the meantime, Tyndall said, the Guyanese government must decide what to do about the financing. One option is to apply for financing under an ordinary capital loan, which carries a much higher interest rate but only requires a simple majority vote for approval.