Prince George's County officials yesterday opened a new jail annex designed to temporarily relieve severe overcrowding at the county Detention Center in Upper Marlboro.

Provision of the two-story, $635,000 building has been cited by jail officials as an improvement that would decrease the likelihood of sexual assaults among inmates. Numerous rapes and other sexual assaults occur in the jail, according to guards and inmates, partly because of the overcrowding problem.

About 30 inmates were moved to the new building yesterday, and an additional 50 are to be moved today, according to jail Lt. Robert Mellin. The 100-bed building is intended to be used for a few years until the county builds a new 300-bed jail.

The decision to build the temporary building was made as part of a settlement agreement between county officials and inmates who filed suit because of overcrowding.

Inmates who tend be the victims of rapes and sexual assaults are usually in the jail on misdemeanor charges and generally are raped by men jailed for violent crimes.

The new facility will be used only for men who have been charged or convicted of misdemeanors and have no prior record of violent behavior, according to Mellin. The top floor will be used for men sentenced to the county's work detail or work release program. The bottom floor will be used for men awaiting trial on misdemeanor charges.

Prince George's voters will decide in a referendum Nov. 2 whether to authorize a bond issue for a new $40 million jail. In settling the inmates' lawsuit last month, county officials promised to build a new jail that would be ready for occupancy by Sept. 30, 1985. If voters reject the bond issue proposal, a federal judge could still force the county to build the jail using money from property taxes or other county money.