With the quiet backing of the Reagan administration, Panamanian opponents of Gen. Manuel Antonio Noriega yesterday moved to increase financial pressures on the country's military rulers by blocking their access to an estimated $50 million in Panamanian government funds held by five U.S. banks.

U.S. officials were reluctant to discuss their role in orchestrating the strategy. But they confirmed that the administration had eased the way for suits against the banks by certifying that it still recognizes Eric Arturo Delvalle, deposed by Noriega, as president of Panama and Juan B. Sosa, who remains loyal to Delvalle, as his ambassador here.

William D. Rogers, a prominent Washington attorney who is advising the opposition, said that as of last night requests to prevent payment of the funds to the Noriega-controlled government had been filed against four New York banks: Republic National Bank, Marine Midland, Irving Trust Co. and Bankers Trust Co.

He added that a separate request for a restraining order had been made against the Bank of Boston in that city.

Rogers, a former assistant secretary of state for inter-American affairs, said that Republic agreed late yesterday to transfer the disputed funds in its possession, estimated by Panamanian opposition leaders to be as much as $10 million, to the Federal Reserve Bank in New York pending resolution of the dispute over who legitimately represents the Panamanian government. He said that hearings on the suits against the other banks are expected today in U.S. district courts in New York and Boston.

In a related aspect of the escalating struggle to force Noriega's ouster, the leadership of the House Merchant Marine and Fisheries Committee sent a letter to President Reagan, urging that the $7 million payment due to Panama March 15 under the 1978 Panama Canal treaties be put "in a special escrow account until the legitimate government . . . is in control." Administration officials indicated that Reagan is likely to agree to the request.

The $7 million is part of the $80 million that the United States pays to Panama annually as part of the treaty arrangements leading up to the transfer of the canal to Panamanian control at the end of the century. U.S. officials said that the $80 million accounts for about 12 percent of the Panamanian government's annual revenues.

The move to freeze funds in U.S. banks is based on an official certification, signed last night by acting Secretary of State John C. Whitehead, that the United States regards Delvalle as the legitimate president of Panama and Sosa as his authorized representative here. That opened the way for Sosa to warn the banks that they are liable for any Panamanian government funds in their possession and to seek to prevent their disbursement to anyone other than him or his agents.

U.S. officials, seeking to avoid the impression of overtly interfering in Panamanian affairs, refused to say whether they would support that argument to the banks. But Elliott Abrams, assistant secretary for inter-American affairs, told Cable News Network:

"If the government of President Delvalle says to American banks, 'Don't pay money to Noriega,' then all those banks are going to come to the State Department and say, 'Who's got the authority here? Noriega or Delvalle?' We're going to answer that question: Delvalle is president of Panama."

The reaction of the Republic National Bank indicated that the strategy was having some success, at least in the short term, of tightening Panama's economy, straitened since last summer as the result of unrest against Noriega.

The military strongman, in control since 1983, has been charged with widespread corruption and was indicted last month by two U.S. federal grand juries on narcotics trafficking charges.

However, some U.S. experts on Panama cautioned yesterday that the strategy being orchestrated from the State Department is a gamble that could backfire.

Several attorneys said there are questions whether U.S. courts would be willing to get involved in the dispute over the funds because, as one put it, "The question legally is not whether Delvalle is the legal president of Panama but whether he is able to invoke the powers of the president."

That source, who declined to be identified, is a former diplomat with extensive experience in dealing with Panama. He noted that the real point of the bank suits rests not on the ultimate decision about their legality but their effectiveness as a pressure tactic to force Noriega's resignation.

"The fallacy is that Delvalle is a weak figure around which to try to rally opposition," he said. "He was a figurehead put in office by Noriega and has no support in his country. He's in hiding, and the attempt to portray him as a heroic leader of the resistance could be seen by Panamanians as an American attempt to use him to impose its will on the country."

Ambler Moss, an attorney and former ambassador to Panama who now is dean of the Graduate School of International Studies at the University of Miami, agreed.

"These financial harassing tactics can work for a few days," he said. "But how long can we base a policy on recognizing one president who is in hiding and one ambassador who is holed up in the Washington embassy? If a few days go by and Noriega is still there without any great groundswell of support for Delvalle, we could be very far out on a shaky limb."

Rogers and Panamanian exile leaders here, such as Gabriel Lewis Galindo, a former ambassador to the United States, took the opposing view, saying that Panama cannot continue too much longer with its parlous financial condition. The country of 2 million owes about $2 billion to foreign commercial banks and about $1.5 billion to international lending institutions, giving it one of the highest per capita foreign debts in the world.