President Reagan yesterday imposed economic sanctions against Nicaragua, saying in Bonn that the steps were taken "in response to the emergency situation created by the Nicaraguan government's aggressive activities in Central America."

In putting a total embargo on trade with the leftist Sandinista government, Reagan also suspended landing rights for Nicaraguan planes and ships and announced his intention to end a Treaty of Friendship, Commerce and Navigation with Nicaragua.

However, State Department officials said here that existing overseas subsidiaries of U.S. firms are not affected by the ban and that other nations were informed of but not asked to join the boycott.

"Only time will tell" how much impact the sanctions will have, Langhorne A. Motley, assistant secretary of state for inter-American affairs, told reporters.

Congressional reaction was mixed, with conservatives applauding the move as a strong step and liberals criticizing it as counterproductive and more damaging to Nicaragua's middle class than to the government.

Nicaraguan officials repeated that they have no aggressive intent toward their neighbors and said the administration has no reason to declare an emergency other than to prepare for war.

Senate Majority Leader Robert J. Dole (R-Kan.) said the sanctions would help Reagan attain his real objective -- aiding the rebels fighting the Sandinistas. Last week, Congress refused to renew assistance to the "contra" rebels but came close enough to doing so that another vote is expected soon.

During that debate, many members of Congress called for economic measures as a way to exert other than military pressure on Nicaragua to become more democratic.

"I think it will help Reagan's case," Dole said. "I think it's time to get tough . . . and stay tough."

Rep. Michael D. Barnes (D-Md.), chairman of the House Foreign Affairs subcommittee on Western Hemisphere affairs, said he would summon administration witnesses to a hearing on the sanctions next Tuesday. He called it "ludicrous" to say the Nicaraguan situation warrants Reagan's statement that he would "declare a national emergency to deal with that threat."

"This is another in the long series of mistakes that have characterized the administration's Nicaragua policy," Barnes said.

White House spokesman Larry Speakes announced the sanctions while Reagan was resting at a guest house of the West German government. He is in Bonn for the seven-nation economic summit.

The White House acted under the International Emergency Economic Powers Act, which requires declaration of a national emergency and a report to Congress every six months on the status of relations.

The move appeared to be the chief, if not only, result of a "broad review" of U.S. policy toward Nicaragua that Reagan ordered last week. Briefing reporters, Motley said no fundamental change in approach had been contemplated.

Speakes said Nicaragua's efforts to "subvert its neighbors" and intensify "communist totalitarian internal rule" had increased since the votes in Congress, Washington Post staff writer Lou Cannon reported from Bonn.

As examples, Speakes cited "new ties" between Nicaragua and the Soviet Union after last week's visit to Moscow by Nicaraguan President Daniel Ortega, apprehension of seven Nicaraguan agents in Honduras, and recent deliveries of helicopters and other military equipment from the Soviet Union and East Germany.

Speakes said Reagan is not abandoning his hope of providing aid to the rebels. "The president continues to believe that direct pressure presents the only effective means of moderating Nicaraguan behavior and is using the means available to him toward that end," Speakes said.

"He urges all members of the Congress to support future requests for assistance to the Nicaraguan democratic resistance. He has also made it clear that the embargo does not apply to those goods destined for the organized democratic resistance," Speakes said. The emergency powers act includes an exemption for humanitarian aid.

U.S. trade with Nicaragua has declined under the Reagan administration, according to State Department figures, from $324 million in 1981 to $168 million last year, as hostilities increased and Nicaragua sought other markets for its bananas, sugar, beef and coffee.

According to those figures, Nicaragua still buys more in the United States than it sells here -- $111.5 million and $57 million, respectively, last year.

Nicaragua's numbers show a larger gap, $157 million to $45.3 million, and its officials argue that the discrepancy is an administration trick to play down the effect of its move on the U.S. balance-of-payments deficit. State Department analysts say the difference results from invoice-juggling in Managua.

"We are disappointed that the administration has gone ahead with economic sanctions without realizing that the effect of these sanctions is not going to be productive for the United States," Nicaraguan Foreign Trade Minister Alejandro Martinez said in a San Francisco speech.

He said the sanction move "violates international law and violates the principles of the General Agreement on Tariffs and Trade" and called the sanctions "nonsensical."

Motley, asked whether Nicaragua could seek GATT countermeasures, indicated that it is possible but was "figured into the considerations" of whether to take the step. He added that the administration consulted fully with its allies and other Latin American nations before deciding to act but has no indication of their reactions.

White House chief of staff Donald T. Regan was asked on NBC-TV's "Today" show whether the sanctions might allow Nicaragua to blame the United States for its "economic mess."

"No, they're already in a mess," Regan replied. "They owe over $5 billion, they have no way of paying it . . . . There's no way they can lay that on us."

The International Monetary Fund puts Nicaragua's external debt at $4 billion, and a Treasury Department official said $92 million of that is owed to U.S. banks. The country's inflation rate is approaching 250 percent annually and shortages of many consumer goods are widespread.

Speakes was asked whether the impact of the sanctions would drive the Sandinistas even closer to the Soviets.

"It's difficult to see how we could drive Nicaragua any closer . . . into the arms of the Soviets," he said. "For six years, they've been totally receptive to Soviet aid."