Washington restaurateur Hassan H. Mohammadi testified yesterday that he smoked opium with Mayor Marion Barry more than a dozen times, and that after the Ramada Inn episode in December 1988 he urged the mayor to reduce his drug use and to be careful with whom he used drugs.

The drug and perjury trial recessed abruptly after Mohammadi's testimony yesterday when prosecutors said they were unable to find their next witness, Theresa Southerland. Southerland, a former girlfriend of the mayor's, is expected to testify that she used cocaine with Barry several times over the last three years.

Assistant U.S. Attorney Judith E. Retchin told U.S. District Judge Thomas Penfield Jackson that Southerland's attorney, Gwendolyn Daniels, could not be reached. Jackson issued a bench warrant for Southerland's arrest.

Southerland, located by deputy U.S. marshals last evening, appeared briefly before U.S. Magistrate Deborah Robinson, who allowed her to go home after being assured that Southerland would appear in court this morning, according to Southerland's attorney, Gwendolyn Daniels.

A spokesman for the U.S. Marshals Service called the affair "a misunderstanding."

Mohammadi, the fourth prosecution witness to testify that he saw Barry use cocaine, told the jury that he brought opium to Barry at social gatherings all over the city. He has testified he introduced Barry to opium in 1988.

In July 1989, he said, Barry invited him to a crab feast at his home. While most of the guests were enjoying the party outside, he said, Barry asked him if he "had anything," which Mohammadi said was their code word for cocaine. They went upstairs to a bathroom off Barry's bedroom and snorted cocaine, Mohammadi said.

Other times, they did it in Barry's basement, he said. Barry is not charged with possessing opium, an illegal drug, but Jackson ruled Monday that the mayor's alleged opium use was admissible to show predisposition to use cocaine. Barry is asserting he was entrapped at the Vista Hotel, and the government will counter the entrapment defense by arguing he was predisposed to cocaine use.

Mohammadi testified that sometimes he and Barry used opium by itself, and other times with cocaine.

Opium is an addictive narcotic derivative of the poppy plant, grown mostly in the Middle East and Southeast Asia. Heroin and morphine are derived from it.

Yesterday, as he did on Monday, Mohammadi demonstrated his technique for using drugs, this time employing a straw to portray smoking opium from the back of a heated spoon.

Defense lawyer Robert W. Mance tried to prevent the demonstration, arguing at a bench conference that it was "highly prejudicial." Jackson said he was "inclined to agree," but prosecutor Richard W. Roberts urged Jackson to allow the demonstration.

Mohammadi described how opium is used, starting with a description of its popularity in his native Iran. The processed drug "looks like a lot of pencils which are brown-colored like chocolate," he testified.

Mohammadi told of using a small plumber's torch to heat a metal clothes hanger that in turn heated the spoon. The heat would vaporize the opium on the back of the spoon's curved surface, said Mohammadi, who testifed that he and Barry used either a straw or the paper tubing from a clothes hanger to inhale the smoke.

During a recess, Barry told reporters they should not believe Mohammadi because pressure was put on him to testify. Barry cited prosecutors' agreement to recommend against deporting Mohammadi back to Iran. "Deportation is a horrible thought," Barry said. "They would cut his head off."

Mohammadi pleaded guilty in May to conspiring to possess cocaine. He has not been sentenced in that case.

Under cross-examination by Mance, Mohammadi said that he had no choice except to cooperate with the prosecution.

After the Vista sting, Mohammadi said, "everybody came from woodwork. Everybody, all of his friends, they cooperate with the government."

"Because everybody else did it, you decided you ought to do it? Is that right?" Mance asked.

"I did not have no choice," Mohammadi said.

Mohammadi said he had been loyal until then to Barry. "I was a true friend to Mr. Mayor," he said. "I was always there for Mr. Mayor."

Barry didn't pay him for the cocaine, the opium, the food, the use of his apartment or Barry's $3,000 in Bahamian gambling debts that he financed, Mohammadi said. Neither prosecution nor defense attorneys asked the witness to explain his generosity.

The FBI has been investigating a Mohammadi company that received a $195,000 marketing contract from the D.C. lottery board.

In his testimony yesterday, Mohammadi said Barry once came to the M Street NW restaurant Mohammadi owned until recently, Pardis Cafe, to meet Southerland.

"Mr. Mayor asked, did I have my spoon," Mohammadi testified, saying Barry wanted to smoke opium. "It was kind of new for Miss T," Barry's nickname for Southerland, said Mohammadi. He said he gave Southerland some cocaine, which he said she snorted with Barry in the apartment above Mohammadi's restaurant.

Mohammadi testified that in early 1988, Barry invited him to the 16th Street NW apartment of Hazel Diane "Rasheeda" Moore. Upon arriving, Mohammadi said, he smelled marijuana smoke, and he said Barry asked him about opium. Mohammadi said he then smoked opium with Barry and Moore.

At one point, Roberts asked Mohammadi about an invitation Barry extended for a visit to a yacht moored in the Washington Channel in February 1989. Sources said the boat was owned by Barry friend Carthur Drake, whose companies have received D.C. contracts worth several million dollars. It was on that yacht trip, about two months after the Ramada Inn episode, that he issued his warning about drug use to Barry, Mohammadi testified.

"What, if anything, did you say to the defendant about his continuing to use drugs at that time?" Roberts asked.

"I told mayor before, you know, at the time to cool it down . . . . You have to know who you are dealing with," Mohammadi said. "And he says, 'Everything is fine. I have done nothing wrong' "

The restaurateur said that Barry was accompanied that day by Southerland, and that when he gave Barry and her a small amount of cocaine, he saw the couple snort it.

Mohammadi also said that in early 1988, he advised Barry not to continue doing drugs with a mutual friend, restaurant manager Sammad Arshadi. Mohammadi said he told Barry that Arshadi "goes around and tells people he did drugs with mayor."

In his cross-examination, Mance sought to portray Mohammadi as a greedy man who turned on friends whenever he thought it necessary to avoid deportation.

Mance referred to a 1982 agreement in which Mohammadi pleaded guilty to fraud and received four years' probation in return for his testimony against others involved in a scheme to bribe an Immigration and Naturalization Service official.

"Every time you get into trouble, you get yourself out by cooperating with the government, isn't that right?" Mance asked. Mohammadi denied it.

Mohammadi testified that he never used cocaine on his own, but only with the mayor "to make him happy." He said he bought the drugs from Arshadi or from someone he identified as "Jeff," who ran a used car dealership on Columbia Road NW.

Activist Dick Gregory attended the morning session. He saidhe was in Washington to attend services for Mitch Snyder and "to show my outrage for this corrupt and vicious system that is racist and sexist."