PANAMA CITY, JAN. 10 -- Military strongman Gen. Manuel Antonio Noriega returned today after a one-day trip to the Dominican Republic that sparked joyful demonstrations at home by opponents who thought he would not come back.

Noriega, who has been under pressure from Washington to ease his grip on power, reportedly said in the Dominican Republic that his unannounced trip was aimed at testing reaction by his opponents at home and in the United States.

"He is in the country, but we cannot inform you in what place. But he is {back} here," said an officer of the Defense Forces, of which Noriega is commander-in-chief.

In the Dominican Republic, Noriega was quoted by the Listin Diario newspaper today as saying his trip was a ploy to see how the Panamanian opposition and the United States would react.

"It was a trap I set to test their honesty," he said.

A State Department spokesman in Washington said yesterday that it was understood that Noriega went for a weekend "family outing."

The Dominican Foreign Ministry had announced Noriega would stay in the country for five days.

But in Panama City, about 1,000 people demonstrated joyfully in the banking district Saturday when Noriega's departure became known. Many were under the impression he was gone for good.

Noriega, who has been accused of involvement in drug trafficking and the assassination of political opponents had said he would go home after visiting the family of a retired Dominican general. One of Noriega's daughters is married to the general's son.

The U.S. government has been pressuring Noriega to implement democratic reforms and hold free elections. Richard L. Armitage, U.S. assistant secretary of defense for international security, met with Noriega in Panama recently to impress the Reagan administration's views upon him.