WASHINGTON, Dec. 18— A confrontation between the police and a prominent church leader in Rumania over the weekend led to a large-scale anti-Government protest that reportedly resulted in many casualties as security forces responded with water cannon, tear gas and some gunfire.

Reporters were not allowed near the scene of the clash in the city of Timisoara in western Rumania, not far from the border with Hungary.

Accounts of the unrest were pieced together here from Eastern European and Western news agencies, which interviewed witnesses who left Rumania in the last two days. Details also came from radio broadcasts monitored by the State Department’s Foreign Broadcast Information Service and from briefings by department officials.

The State Department said today that the United States Embassy in Bucharest, the Rumanian capital, which is about 500 miles from the area of the unrest, still could not confirm reports of casualties. The witnesses’ accounts greatly varied – with the number of dead as low as two and as high as several hundred.

The demonstrations were the first reported in Rumania since people in East Germany, Czechoslovakia and Bulgaria began taking to the streets in recent months in peaceful uprisings that forced their Communist rulers to share power.

They apparently began as a result of ethnic tensions between the hard-line Communist regime of President Nicolae Ceausescu and members of the country’s Hungarian minority, which numbers about two million.

State Department officials and East European press agencies said the protests took on a broad anti-government character like those elsewhere in Eastern Europe. The official Yugoslav press agency said the police used guns and water cannon against as many as 10,000 protesters in Timisoara, who shouted ”Freedom!,” ”Rumanians rise up!” and ”Down with Ceausescu!”

”It looks like Rumania’s time may have finally come,” said a State Department analyst. ”I would emphasize the word ‘maybe’ though. Ceausescu has a pretty tight lid on the place. But it will happen sooner or later.”

The American Ambassador to Rumania, Alan Green Jr., was instructed by the State Department to lodge a complaint with Mr. Ceausescu’s Government, long considered the most repressive in Eastern Europe after that of Albania. Police Attacks Reported

Witnesses interviewed by Hungarian and Yugoslav press agencies said the clash began when the police attacked crowds trying to block the eviction of a popular ethnic Hungarian priest and human rights campaigner, the Rev. Laszlo Tokes. Witnesses who arrived tonight in Budapest told Hungarian television that the confrontation quickly turned into a demonstration against Mr. Ceausescu, with thousands of people taking part.

The State Department spokeswoman, Margaret D. Tutwiler, said several hundred members of Mr. Tokes’s congregation apparently mounted a demonstration on Saturday ”in support of his refusal to vacate the church premises.”

”The demonstration apparently grew in size and took on an anti-Government tone,” Miss Tutwiler said. ”On Sunday, Dec. 17, Government security units brutally put down the demonstration with the use of truncheons, tear gas and water cannons.”

A witness quoted by Hungarian television said a fire truck was set ablaze by demonstrators and another said the crowd marched on city hall, shouting that the Rumanian people should take note of what was happening in Czechoslovakia, Hungary and Bulgaria. No Food, Pregnant Woman Says

Another witness said a pregnant woman pointed at her belly and told soldiers to shoot her there because there was not enough food in the stores anyway to feed her children.

Yet another witness said she heard of similar protests taking place in Arad, a town near Timosoara.

American Embassy personnel have repeatedly tried to visit Mr. Tokes, Miss Tutwiler said, but have been barred by security forces. ”We call upon the Rumanian Government to cease its pressure on Reverend Tokes and his congregation and allow him to exercise his fundamental freedom to communicate,” Miss Tutwiler said.

Hungarians are the largest minority in Rumania. For years they had had their own schools and cultural centers, were allowed to speak their own language and were freely promoted in the Government bureaucracy. But the Ceausescu leadership has gradually withdrawn these privileges. As Hungary has liberalized its policies and opened up to the West in recent years, Mr. Ceausescu has became more suspicious about ethnic Hungarians in his country.

He apparently felt he had the situation well enough in hand to leave Bucharest today for a three-day state visit to Iran.

The Associated Press said that traffic into Rumania was reported virtually halted on the Hungarian, Yugoslav and Bulgarian borders. It said that there were also several reports of Western flights being turned back at Bucharest airport, which diplomats said was under tight security.

Charles Gati, an expert on Eastern Europe at Union College, said it was logical that the Rumanian Government would use violence to suppress demonstrations.

He said the leadership, which is dominated by the Ceausescu clan, has managed to largely purge its intelligence services of Soviet agents, unlike the intelligence services of other East European countries. Mr. Gati said that as a result, the ability of the Soviet Union to encourage gradual reform, beginning in the Communist Party, is much more limited in Rumania. Violence Foreseen

Nevertheless, Mr. Gati said: ”Whatever you may think about the domino theory in Asia in the 1960’s, it is certainly working in Eastern Europe in the 1980’s. It will be impossible for Ceausescu to maintain a Stalinist regime when even the Brezhnevite regimes have fallen all around him.”

”If change is to come to Rumania, it will almost certainly have to be violent,” he continued. ”It cannot happen peacefully like everywhere else.”

State Department officials said it was difficult to confirm any casualty figures or to provide a detailed version of events, because of the distance between Bucharest and Timisoara, a city of perhaps 200,000 people.

The most severe account of casualties came from Radislav Dencic, a graduate of Timisoara University who was in the city for a week and returned to Yugoslavia today. He was quoted by the Reuters news agency as saying that Rumanian security forces shot at the protesters from the street and from helicopters.

”Hundreds of people were falling on the pavement in front of my eyes,” he told reporters.

Photo of a womaan crossing the border from Hungary after a shopping trip (Agence France-Presse); map of Rumania showing location of Timisoara (NYT) (pg. A16)