The Soviet flag had been flying at half-mast from the mansard roof of the Soviet Embassy since 8:40 a.m. Little else seemed different there yesterday, nothing charged, nothing hurried. The slate-gray shutters on the top two floors were closed and the curtains on the bottom two were drawn, but that is almost always the case.

As the afternoon rush hour began and the rain started to fall, Secret Service officers cut off traffic in front of the embassy on 16th Street downtown. First, Secretary of State George Shultz arrived, then, at 5:30 p.m., the president's limousine pulled up to the door.

Inside, President Reagan expressed his sympathies to Ambassador Anatoliy Dobrynin, who wore a red armband. Near a photograph of the late Soviet leader Konstantin Chernenko framed with black crepe and flanked by two Soviet flags, Reagan wrote in a book of condolences: "My condolences and sympathy to Chairman Chernenko's family and to the Soviet government and people in this time of bereavement. Let us rededicate ourselves to ensuring a lasting peace between our countries." Soviet military officials stood to the right of Chernenko's photograph and party officials stood to the left. They, too, wore red armbands.

Five minutes after arriving, Reagan stood with Dobrynin on the embassy steps. They stood there long enough for the photographers to take pictures but not long enough to be soaked by the rain. After shaking hands with Reagan, Shultz and White House chief of staff Donald Regan, Dobrynin waved, waited until the motorcade pulled out onto the street, then went back inside. One White House official said the scene was "identical" to the one 13 months ago when Yuri Andropov died.

The rituals of Soviet mourning have become strangely familiar. In the last 28 months the Soviet Union has experienced three changes in leadership, losing Leonid Brezhnev, Andropov and now Chernenko. According to embassy spokesman Mikhail Lysenko, many more leaders and diplomats will visit the embassy today.

"But we are conducting our business activities as before," Lysenko said. "Of course, people feel sorrow for the death of our leader, but that's the only change, really." The embassy school on Tunlaw Road was open, but it will be closed on Wednesday, the day of the funeral.

In the morning hours, as the White House was deciding who would represent the United States at Chernenko's funeral, a few people stopped to gaze at the embassy, a four-story French classical revival town house. Eugene Lange of Tomball, Tex., clutched a Pentax to his breast and looked across 16th Street. He had never seen the embassy before.

"You see all the antennas on top of that building and you look at the guy who came out just before in one of those funny Russian hats and you wonder what's going on in there and in Moscow, who they're going to come up with," Lange said to his wife Jo.

The usual crew of uniformed Secret Service officers watched the usual come-and-go at the black wrought-iron gates. A delivery man, Oji Pierce, stopped by to drop off an envelope. "There's hardly anybody in there, nothing unusual," Pierce said on his way out. "Just a few visitors."

A beige Econoline van pulled up. Lawrence Lee of Palace Florist on Dupont Circle delivered four floral arrangements ordered by the embassy: three bouquets of red carnations and an immense spray of three dozen long-stemmed roses. The white urn that held the roses was wrapped in a wide black ribbon.

"We came down here when the last guy died," said Lee. "Same deal, same flowers."

Lange brought his camera to his eye. He and his wife had already snapped pictures at the Lincoln, Jefferson and Vietnam memorials, John F. Kennedy's grave and the White House.

"But this is special, isn't it?" Jo Lange said.

With his film exposed, Eugene Lange took her hand and started walking away from the embassy. "I've never had a desire to visit Russia as such," he said. "It's a little too cold. This was kind of an event for us, though."