Two Maryland men under arrest in the bombing of a series of abortion-related facilities had discussed the "preparation for the bombing" of similar offices on the day they were arrested, according to indictments handed up here today.

A federal grand jury indicted the men and another man who was also arrested Jan. 19 on charges of conspiring among themselves and unspecified "others" in the bombings and arsons of 10 abortion clinics and related facilities in Maryland, Delaware, Virginia and the District of Columbia.

The arrest of the men -- Thomas Eugene Spinks, 37, and Michael Donald Bray, 32, both of Bowie, and Kenneth William Shields, 34, of Laurel, culminated an intense investigation by local and federal law enforcement agents on the eve of the annual Jan. 22 protest march in Washington against the 1972 Supreme Court ruling legalizing most abortions.

Today's indictments said Bray and Spinks discussed the "preparation for the bombing of abortion-related facilities" in a telephone conversation on Jan. 19. The indictments did not say whether a specific target was mentioned.

Federal officials have said they were concerned that militant foes of abortion might attempt a spectacular bombing in downtown Washington, marring not only the abortion protest march but also the planned inaugural parade on Jan. 21 for President Reagan. Reagan, although a supporter of the antiabortion movement, recently had condemned bombings and other violence by small factions within the movement.

The lengthy indictments, released by prosecutors here, accuse the three men of planning the bombings and arsons during 1984 and early this year, buying explosives and metal cylinders for pipe bombs and "conducting surveillance" at two clinics in preparation for bombing them.

The indictments allege that Spinks by himself set off the explosions or fires at each of the 10 sites after he and Shields purchased large amounts of chemicals and expended fire extinguisher cylinders for pipe bombs.

The indictments said Spinks, Shields and Bray conspired not only among themselves but also with "others known and unknown to the grand jury." Prosecutors have said the investigation is continuing and more arrests are possible.

Last year's bombings and arsons -- most of them triggered at night, caused thousands of dollars in damage but no injuries.

Agents from the federal Bureau of Alcohol, Tobacco and Firearms and other law enforcement officers have said they linked the 10 incidents because of similar ingredients found in bomb fragments at the blast sites, and traced purchases of chemicals, fuses and other components to Shields and Spinks and the surveillance of two abortion clinics to Bray.

All three men have been described by law enforcement agents as fervently religious opponents of abortion. Spinks, a roofer and chimney sweep, is being held without bond pending trial. Bray, a housepainter and copastor of the Reformation Lutheran Church in Bowie, is free on $250,000 bond, and Shields, comptroller of a Warrenton, Va., firm, is out on $25,000 bond.