When a team of American election observers led by Sen. Richard G. Lugar (R-Ind.) left a hotel here this morning en route to the airport for a flight home, about 50 demonstrators were waiting at the gate.

The Americans, who had just issued a statement condemning fraud in Friday's Philippine presidential election, passed through in cars without incident. The protesters waved placards reading "Down with U.S. Imperialism!" and "Sen. Lugar, CIA Terrorist," then burned a U.S. flag and chanted slogans against imperialism.

Despite the leftist appearance of the demonstration, there was speculation that the group was brought to the government-owned Manila Hotel by a wing of President Ferdinand Marcos' ruling New Society Movement. Men in charge of the group tried to block contact between reporters and demonstrators. But one woman said they were from Marcos' party.

The reception Americans enjoy in the Philippines, a U.S. colony until 1946, is about as warm as they find anywhere in the world. But at times of tension between the two governments, the cork comes off the bottle of anti-Americanism. Some of it is resentment by citizens of a smaller country against its former lord. But much seems to be generated by the government to give a reminder that it has options other than the close alliance with the United States.

Few analysts expect that Marcos, now in his 21st year in power, would ever make the break. Still, his government never stops talking of it, especially when the question of U.S. military bases here comes under review and Manila seeks to raise their rent.

Under an agreement in 1983 for the use of the Subic Bay Naval Base and Clark Air Base, the United States is committed to seek $900 million in aid over five years beginning in fiscal 1985. In the current fiscal year, Congress approved $184 million, of which $54 million is in military aid.

When relations with Washington reached a critical stage last fall, the progovernment press gave prominent play to Marcos' wife Imelda being honored during a tour of the Soviet Union.

Pressure from Washington is increasing again over Marcos' conduct of the presidential election. President Reagan has said he would consider a substantial increase in aid if the elections were fair and credible to the Filipino people. But Lugar said the group had found "disturbing reports" of electoral abuse and would report about them to Reagan Tuesday. And once again, publicly expressed sentiments against Americans here are on the rise.

When newspapers reported that two U.S. aircraft carriers had called at Subic Bay Naval Base, columnists quickly fired off charges of gunboat diplomacy and efforts to influence the election. U.S. officials said the vessels' presence was routine.

Marcos' political affairs minister, Leonardo Perez, last month called U.S. Ambassador Stephen Bosworth a "self-annointed American praetorian guard of democracy in the Philippines." He urged him to "shed his mantle of hypocrisy and be man enough to mount the campaign stage" with opposition candidates.

Ruling party spokesman J.V. Cruz, formerly Marcos' ambassador to Britain, often leads the criticism. Lately, he has attacked the U.S. endorsement of the citizen poll-watching group known as Namfrel, which Marcos has accused of distorting election returns to favor Corazon Aquino.

Last week, Cruz said U.S. support had "finally and conclusively" revealed "the umbilical cord that has always tied Namfrel to colonialist elements in the United States."

Progovernment columnist Teodoro Valencia can be counted on to take a swing at Americans almost every time he writes.

Marcos generally keeps above the fray. He talks often of a need to maintain cordial relations but says he would bow to the inevitable should the Americans break off. "What can we do do?" he asked last week. "We have to reach a modus vivendi with other powers in the region."