An army of 20,000 citizen-volunteers joined city employees in Detroit in patrolling the streets to combat the outbreak of arson that has become traditional on the night before Halloween, dubbed Devil’s Night. The volunteer force, including members of 46 citizens band radio clubs, block clubs and other organizations, was twice as large as the group of volunteers deployed in the last few years, said Robert Berg, a spokesman for Mayor Coleman A. Young. Those volunteers joined 750 Department of Public Works employees and 150 department heads, staff members and other mayoral appointees patrolling Detroit streets in city vehicles equipped with fire extinguishers, Berg said. At least one building went up in flames in the afternoon and another blaze razed an abandoned residence and spread to an occupied home, but there were no reports of injuries. Police picked up 370 minors on curfew violation charges. Last year, police arrested 496 youths on similar charges.