President Ferdinand E. Marcos said today that a cancellation of President Reagan's visit here could be a blow to the Philippines and would raise new political problems about operation here of two key U.S military bases.

Marcos said in an interview that canceling the visit would be "unthinkable" and would demonstrate that the United States does not consider the Philippines important. He predicted it would revive political opposition to the bases, make it difficult for him to implement a new base-leasing agreement, and have "severe" economic repercussions for his country.

Meanwhile, what was to have been a rally in support of Marcos' beleaguered government broke up in violence and chaos in Manila's financial district in suburban Makati when thousands of anti-Marcos office workers threw rocks, bottles and water bombs on the marchers from windows of high-rise buildings.

Those attempting to break up the progovernment rally shouted "Marcos resign!" and "Ninoy, Ninoy!" the nickname of Benigno S. Aquino Jr., the Philippine opposition leader who was assassinated Aug. 21 after his return here from the United States.

Marcos directly linked the issue of the Reagan visit and the American naval and air bases during the interview with four American reporters, amid reports that the Reagan administration is reconsidering the president's planned one-day stopover in November.

The U.S. administration is said to be concerned about Reagan's appearance here in the aftermath of the slaying of Aquino.

No one has been arrested in the killing and there are widespread suspicions here that someone in the government ordered it. An official investigation has foundered amid legal challenges to its independence and reliability.

The United States reportedly has put pressure on the Marcos government to come up with a satisfactory investigation and there are reports that it has used the possibility of canceling Reagan's visit as a tactic.

The Asian Wall Street Journal reported today that U.S. Ambassador Michael Armacost has told the Philippine government that under present circumstances he could not recommend that Reagan come here.

Armacost refused to comment on the report today. An embassy spokesman denied that he had made a recommendation to the State Department to cancel the visit. But the spokesman said he could not comment on what Armacost might have said in private meetings with the Philippine government.

Marcos, in the interview, said he had not been notified of any changes in Reagan's plans and said he assumed the state visit would take place as planned.

A source close to Marcos, however, said the government has been informed that the White House is concerned that supporters of Aquino would mount embarrassing demonstrations against Reagan if he comes.

The U.S. military bases Marcos referred to are Subic Bay Naval Base and Clark Air Force Base, both important parts of the American military presence in the Pacific and the Indian oceans. Marcos recently agreed to a new five-year American use of the bases in exchange for $900 million in U.S. security assistance.

Today Marcos insisted that he would continue to support U.S. use of them even if Reagan canceled his visit but he repeatedly predicted that a cancellation would make his political problems difficult.

Canceling the visit, he said, would cause "the opposition to once again create an uproar against the bases. I'd have to go back to square one and explain the reason for them."

Marcos said that one of the effects of a Reagan cancellation would be that it would be taken to mean that the United States no longer considers the Philippines an important ally. He pointed out that the bases are the source of U.S. military strength in the Straits of Malacca and the South China Sea.

The economic repercussions of a cancellation, he predicted, would be "severe" because it might cause foreign banks to shut down loans and encourage foreign companies to curb their investments here.

The interview today was one of several Marcos has given this week as he has made himself available to foreign correspondents in an attempt to take the offensive against attacks on his government arising from the Aquino assassination. He has repeatedly accused the foreign press of spreading accusations of his opponents here that his government was involved.

One purpose of the appearances seems to be to show that he is in good health and in control of the government. There are widespread reports that he is seriously ill and a succession struggle is under way.

Today, Marcos appeared tired but healthy for a man of 65. He jested at times with reporters who gathered around a large table in a guest house near Malacanang Palace.

He repeatedly assailed the political opposition for questioning the government's role in the Aquino murder and for doubting the effectiveness of an investigative commission he appointed. He called them "gossip-mongers and scavengers" but also repeated his promise to appoint to the commission members suggested by Aquino's supporters.

Washington Post special correspondent Abby Tan reported:

What was intended to be a rally for the Marcos government in metropolitan Manila's financial center turned into a pro-Aquino demonstration as scuffles broke out between government supporters and workers trying to halt their march.

One man was seen bleeding from the head, apparently hit by a thrown bottle. The windows of a government bus were smashed when the driver ignored orders not to enter the street.

Nemesio Yabut, mayor of Makati, the municipality where the financial district is located, organized the rally to counteract the spontaneous display there last Friday of widespread anger over Aquino's murder.

But the government rally lasted less than an hour. Hundreds of schoolchildren and their teachers, bused in by the government, had hardly marched two blocks when fruit, cans, rocks and bottles began flying from windows.

Yabut had begun speaking with a bullhorn as his men shouted "Marcos! Marcos!" Office workers in surrounding buildings countered with: "Ninoy! Ninoy!"

When a water bomb nearly hit Yabut, aides hurried him off.

Police kept a discreet distance and made no move to interfere even as the crowd taunted them.

The government, meanwhile, has designated tomorrow, the 11th anniversary of the imposition of martial law by Marcos, as a day of thanksgiving and ordered village leaders to hold "meet-the-people" sessions to gauge feelings. But the opposition has called it "a day of national sorrow" and planned a noon rally in downtown Manila.