A D.C. police helicopter that had reported mechanical difficulties crashed near the Jefferson Memorial last night, striking a tree and injuring the pilot and observer, authorities said.

Neither of the officers was described as seriously injured in the 8:43 p.m. crash, and authorities speculated that the tree might have broken the fall of the disabled craft.

Deputy Police Chief Isaac Fulwood Jr. said the two-seat helicopter was returning to its base from routine patrol when it radioed the police communications center to report mechanical difficulties. Shortly afterward contact was lost and witnesses reported seeing the craft plunge to the ground.

It "came across the Tidal Basin real low," said Bob Davis of Silver Spring. Then, he said, the engine seemed to stop, and the helicopter "went down real hard."

He said he heard it "go crunch" as it struck a tree between the Jefferson Memorial and the approaches to the 14th Street bridges.

"These men were very lucky," said Lt. Robert Morris, head of the helicopter unit.

In 16 years of D.C. police helicopter operation, at least five other crashes have occurred. In 1979 two officers were killed when their craft crashed and burned in Southeast.

There was no fire last night, but firefighters, working under floodlights, sprayed foam on the wreckage as a precaution. Traffic in the area was delayed for hours.

The pilot of the helicopter was flown by U.S. Park Police helicopter to the MedStar unit of Washington Hospital Center. Officials there described his injuries as abrasions and listed him in good condition. Police identified him as Joseph M. Ruth, 32.

The observer was taken by D.C. police ambulance to the George Washington University Medical Center. Officials there said he suffered a sprained leg. They identified him as Ken Boone, 33.

The Bell 47 helicopter that crashed last night was one of five helicopters used by the D.C. police and based at National Airport. Details about the cause of the crash were not available and the incident is under investigation.