The yen dropped past the 260-to-the-dollar mark in heavy trading here today, reaching its weakest level in 27 months. Some dealers predicted it would continue to fall.

"We're now looking at the 265 target," said one dealer at a major Japanese bank. "But we think the movement will be gradual, step by step."

News of the new decline of the yen prompted Satoshi Sumita, governor of the Bank of Japan, which is the equivalent of the Federal Reserve Board, to say that the bank would intervene "positively and promptly" to support the yen.

Trading began this morning in Tokyo with the dollar at 260.00 yen. It touched 260.90 shortly before noon and closed at 259.85. Intervention by the bank in the morning, profit-taking and a cooled mood helped hold the dollar at the 260-yen level, market analysts said.

Sumita and market analysts here attributed the new fall partly to expectations that interest rates will rise in the United States. As a result, people are said to be selling their yen to buy dollars for investment there.

General optimism in the U.S. economy here because of the past year's recovery also is cited.

In Frankfurt, the dollar set records for the second day running, closing at 3.2205 marks, up from 3.2005 Monday, and in New York it finished at 3.2210, up from 3.2150, United Press International reported.

In London, the pound retreated but still managed to stay on the sidelines in the selling of major currencies in favor of the dollar, closing at $1.1115, down from $1.1138; in New York, it finished at $1.1151, down from $1.1168.

Gold rose $2 an ounce in Zurich to $302.50, and gained 50 cents in London to $302.25. Republic National Bank in New York closed cash gold at $302.25, the same price as Monday. The New York Commodity Exchange settled the February contract at $302.30, up from $303.10.

Republic quoted silver at $6.18 an ounce, up from $6.16 Monday; the Comex settled it at $6.182, up from $6.159. Earlier in Zurich, silver closed at $6.20 and in London it finished at $6.195, UPI said.

The yen has been losing value steadily against the dollar since early 1984, when it took only about 225 to buy a dollar. In the past six weeks, the decline has gained momentum, bringing it down from about 246 on Dec. 19.

Because the weak yen makes Japanese products cheaper abroad, it is blamed by the United States for much of its $34 billion trade deficit with Japan in 1984. Strengthening the currency is one of the prime U.S. policy objectives in relations with Japan.

Last May, the two countries signed a detailed agreement to liberalize Japan's capital markets. That move is supposed to drive the value of the yen up eventually by making it more of an international currency and by increasing demand for it.

U.S. officials have charged that Japan is dragging its feet on implementing the agreement. The Japanese counter by saying that the process is slow and painful but that they are moving ahead.

Bank of Japan Governor Sumita told reporters in Osaka today that his bank's intervention was based on agreement reached in Washington last month by financial officials from Japan, the United States, France, West Germany and the United Kingdom to stem the rise of the dollar against the other major currencies.

But he and Finance Minister Noboru Takeshita ruled out raising Japan's own interest rates to stem the pressure on the dollar. "Due to the need to stimulate the domestic economy, we cannot raise Japanese interest rates now," Takeshita was quoted as saying today.

U.S. government securities currently yield about five percentage points more than Japanese ones. This, coupled with the relaxation in recent years of exchange controls here, has led Japanese to invest billions of dollars in the U.S. market.

Japanese officials contend that the key to reducing pressure on the dollar is to cut the U.S. federal government's deficit and control interest rates in the United States.