BY ELINOR J. BRECHER

  The Haitian town of Thomonde, where a medical mission affiliated with the University of Miami runs a clinic, was seized briefly by anti-government forces Monday, according to the former UM AIDS researcher who oversees the clinic.

  Marie Chery, who directs Project Medishare in Thomonde, said the gunmen welded shut a bridge on the only road leading south, then left.

  POLICE `WITHDREW'

  ''They came and then nobody sees them,'' said Chery. ''That's the anxiety of these things.'' The ''10 to 12'' policemen in the town of 39,000 ''withdrew,'' she added. Chery was interviewed Tuesday by telephone from Port-au-Prince. She was at the home of a friend in the Haitian capital, where she had been marooned since Monday with three Peace Corps volunteers and a shipment of supplies destined for Thomonde.

  She said the former mayor and operators at the city's telephone station had passed on news of the takeover.

  Thomonde is about 15 miles southeast of the town of Hinche, where gunmen fighting to topple President Jean-Bertrand Aristide killed three police officers during an attack Monday.

  Chery believes that Thomonde's residents will safeguard the clinic and Project Medishare's guest house, two of the city's few buildings with electricity and indoor plumbing.

  KEY EMPLOYER

  Medishare, co-founded by UM doctors, is the city's biggest employer. It brings medical students to the dirt-road city several times a year for rural health clinics.

  UM President Donna Shalala accompanied a group in December.

  ''The people are very loyal to the organization,'' Chery said. ``They have invested in it as much as everybody else. It's there for their own people.''

  Dr. Barth Green, Medishare co-founder and head of UM's neurosurgery department, said he is ''terrified for the people we love'' in the region and concerned about the
  mission's facilities.