Soviet television Tuesday showed the explosion of the shuttle on its nightly news program, and Soviet citizens joined leaders around the world in expressing shock and sadness.
The Tass news agency reported the explosion within minutes of the disaster, in a speed rare for the official Soviet news service.
The nightly television news, Vremya, broadcast a videotape of the Challenger, showing the wreckage falling to the Atlantic Ocean.
After the program, a Soviet office worker said, ''I think it's very sad. Any human losses in this field, whether in the Soviet Union or America, are a tragedy. Space exploration is a risky business, but such crashes are a small tragedy for mankind.''
A Soviet woman who also watched the show said, ''I am horribly sad. I feel so sorry for them. It's just horrible.''
There was no immediate comment from Soviet leader Mikhail Gorbachev or Soviet space officials.
But in Washington, Soviet Embassy spokesman Boris Malakhov said, ''On behalf of the embassy, I express deep condolences and sympathy to the American people in connection with this enormous tragic accident involving the shuttle Challenger.''
Soviet Ambassador Vasily Safronchuk, a high-ranking official at the Soviet U.N. mission in New York, expressed ''my mission's deep sorrow'' and conveyed ''to all concerned, particularly families of crew members, our deepest sympathy.''
Leaders from around the world sent condolences to U.S. officials Tuesday after watching television broadcasts of the shuttle disaster.
In Ottawa, Canadian Prime Minister Brian Mulroney said he asked U.S. Ambassador Thomas Niles to convey Canada's ''great grief at the tragedy that has just struck the United States, a terrible loss in remarkably tragic circumstances.''
French President Francois Mitterrand, in a telegram to Reagan, said, ''The French people felt, at the announcement of the explosion of the space shuttle Challenger, a profound emotion and sincere compassion for the astronauts aboard.''
Patrick Baudry, the French astronaut who traveled into space aboard the shuttle Discovery last June, said, ''I think the sacrifice of my friends who were on board will not be for nothing. There have been other accidents before. They have all served some purpose.''
In Pretoria, South Africa, President Pieter Botha said, ''All South Africans were stunned by the tragic news of the explosion. The free world has followed the United States space program with pride.''
Indian Prime Minister Rajiv Gandhi telephoned Reagan and expressed ''his deepest condolences and sympathies,'' the Press Trust of India news agency said.
King Hussein of Jordan sent a cable to Reagan asking the president to convey Jordan's sorrow to the families of those who died in the accident.
The U.N. Security Council paused at the start of debate in New York to extend condolences to the United States.
The European Space Agency said it felt ''deep sympathy with NASA. The accident was a disaster for NASA and for space in general, and as ESA is in space, a disaster for us.''
Ernst Messerschmidt, one of two West German astronauts who flew in Challenger in November, said, ''It is terrible to see these pictures, especially when you know some of the people involved. It's even harder to think that it could have been you.''
Mexican astronaut Rodolfo Neri Vela, who flew on the shuttle Atlantis in November, said, ''I could not accept the idea that this could happen. This is a national tragedy for the United States . . . but I believe it is a tragedy for the world as well.''