A former captain of the Boston Fire Department's arson squad and a retired state police arson detective were among more than a dozen persons arrested yesterday in what officials called the most extensive arson-for-hire ring ever uncovered.

"The magnitude of the case, as it developed, shocked us because of the number of respectable people involved," said an aide to state Attorney General Francis X. Bellotti.

A Suffolk County grand jury returned 120 secret indictments against 33 persons last week, charging then with crimes ranging from arson, fraud and conspiracy to mainslaughter and murder.

In predawn raids in the Boston area, state troopers arrested 14 of the 33 - the former arson agency officials, three public adjusters, two lawyers and seven businessmen. Public adjusters are professional negotiators who represent property owners in working out settlements with insurance companies.

He said the indictments stemmed from a four-month-long investigation of 150 suspicious fires in Boston, suburban Revere and Chelsea, which is adjacent to Boston, between 1973 and 1976.

Three persons died in the fires, and four of the persons arrested were indicted on murder chares in the deaths. Bellotti would not identify those charged with murder.

At an arraignment yesterday, 11 of those arrested pleaded innocent to arson and related charges. They were freed on $100 personal reconizance and their cases were continued until early next month. The others were to be arraigned later.