CHICAGO--Most of Chicago's 4350 firefighters defied a court order and walked off the job yesterday in the first strike in history by Chicago firemen.

Mayor Jane M. Byrne called yesterday "a sad and sick day" for the city, as the walkout left fire protection for 3.5 million people up to a few non-strikers, supervisory personnel, Fire Academy cadets and other city workers, with help from suburban fire departments.

Officials reported no fires through early afternoon yesterday.

The main issue in the dispute was the city's refusal to grant firefighters a written contract instead of the traditional hand-shake agreement unless the union agreed to a no-strike clause and binding arbitration.

Byrne called the situation "a crisis" but promised that "the city of Chicago would meet its responsibility to its citizens and protect their lives, property and general welfare."

The strike began three days after public school teachers ended a two-week walkout to protest layoffs and delayed paychecks. Last December, Chicago Transit Authority workers walked out for several days in a contract dispute.