A Northeast Washington man was sentenced yesterday to serve at least 20 years in prison for his involvement in a string of robberies that took place inside District churches.

Gregory Williams, 33, of 401 Edgewood St. NE pleaded guilty earlier this year to three charges of armed robbery and one count of assault with intent to commit robbery while armed.

D.C. Superior Court Judge George H. Revercomb ordered Williams to serve concurrent terms of 15 years to life on the robbery counts. Revercomb set a term of five years to 15 years for the assault charge, to run consecutively to the other sentence.

Williams must serve 20 years before he becomes eligible for parole. In addition, his sentences in the District will run consecutively to a 60-year term imposed on him in Maryland for similar offenses committed there.

A second man, John Lucas, 32, of 3035 Stanton Rd. NE, pleaded guilty earlier to two counts of armed robbery in two incidents in the District. He was sentenced to 15 years to life.

According to Assistant U.S. Attorney F. Joseph Warin, the government would have proved the following had Williams been tried:

* On Nov. 20, 1981, Williams entered the Mount Horch Baptist Church at 2914 Bladensburg Rd. NE and, armed with a handgun, robbed nine persons holding a Bible class.

* On Jan. 5, 1982, Williams robbed members of a men's club meeting at the Episcopal Church of Our Savior at 1616 Irving St. NE.

* On Jan. 7, 1982, Williams, armed with a handgun, robbed choir members of the Reid Temple AME Church at 1335 Michigan Ave. NE.

* On Jan. 23, 1982, Williams entered the Rhema Christian Center at 4915 Sargent Rd. NE ostensibly to speak to the pastor, then robbed him at gunpoint.