The disruptions of subway service, and dislocations for hundreds of thousands of commuters, caused by a devastating electrical fire under Church Street will probably last for years to come, officials said yesterday.

The fire, which raged from early Sunday afternoon into the evening, short-circuited wiring in a crucial signal-relay control room at the station at Chambers Street in Lower Manhattan.

Officials are investigating the fire as an act of arson. It started after a homeless man ignited a shopping cart containing wood and garbage, according to a press report.

“It has been ruled arson. Everything else is hearsay,” a spokesman for the New York Fire Department, Brian Nolan, said.

Service on the C line, which runs from Washington Heights in Upper Manhattan to Euclid Avenue in Brooklyn, was suspended indefinitely, and transit officials warned that service on the A and C lines may be disrupted for three to five years and cost millions of dollars in repairs.

Transit officials said service on the A line will be slashed by more than half for the next couple of weeks while repairs are made to the signal room. Until signal transmission can be completely restored, the A train will be the only line to service the station and may not return to full service for up to a year, leaving hundreds of thousands of commuters in East New York and Ocean Hill-Brownsville to cram into crowded trains.

Together, the A and C lines have an average weekday rider ship of more than 400,000 passengers. The V train, which usually runs from Forest Hills to Second Avenue at the Lower East Side, will replace service on the C line in Brooklyn, making all local stops between Jay Street and to Euclid Avenue. The A train will run express between Brooklyn and 145th Street in Manhattan.

The additions will probably do little to alleviate overcrowding and delays, the chief transportation officer for New York City Transit, Kevin O’Connell, said. He advised Manhattan-bound straphangers to avoid transferring to the A train from the J, Z, and L lines in Brooklyn.

The Metropolitan Transportation Authority is assessing the damage to the station. MTA officials, who have said the agency is financially strapped, said the repairs would probably cost millions of dollars.

Fire officials said the fire, after starting in a shopping cart, ignited overhead cables and spread into the signal room through the insulation ducts, destroying relays, switches, and circuits essential to transmitting information about train traffic.

The fire department responded to the call at 2:22 p.m. Sunday with 12 units and 60 firefighters. Most of the damage occurred before firemen were able to cut off power to the station. Fans that normally air out the station also made matters worse, and indeed the smell of smoke persisted at the station yesterday.

For now, transit officials are monitoring the trains and operating the signals manually, Mr. O’Connell said. Officials would not say when service to the C line might be restored.

“Service on the A line has always been a major problem,” a Bedford-Stuyvesant resident, Annie Weinmayr, said. “I usually have to wait in the morning for a couple trains to pass because they are full, and sometimes I can’t get a seat at night.”

The A and C trains were not the only lines to experience delays yesterday. During rush hour Monday morning, thousands of riders at Park Slope, Carroll Gardens, and Windsor Terrace were left without F train service because of a frozen rail switch at the station at Smith and 9th streets.

The switch, which routes trains between the express and local tracks, was found buried under several feet of ice and snow early Sunday morning, leading to a suspension of service between Jay Street-Borough Hall and Church Avenue on the F line and parts of the G line.

Additional bus service on the B67 and B75 provided transportation between stations on the F line, but many commuters opted to forgo the crowded buses and brave the slush by walking to the Jay Street station in downtown Brooklyn.

“It was pretty much a single-file line of people all the way down to Jay Street,” Heather Aschinger said. She lives at Carroll Gardens, near the Carroll Street stop on the F line.

“It took about 25 minutes,” she said. “The buses were full of people, and I was walking faster than the bus.”

Transit workers used snow blowers and ice picks to free the switch, but, because of “unusual weather conditions,” restoring service took more than 24 hours; trains were running by about 4 p.m., a transit spokesman, Charles Seaton, said.

“This has never happened before,” Mr. Seaton said. “It was a very stubborn ice condition.”

While commuters trudged through the snow, Brooklyn car services responded to a deluge of calls from residents seeking transportation.

“We can’t answer the phone fast enough,” a dispatcher at Eastern Car Service, Carlos Lopez, said. “It is impossible to travel from one place to the other in this weather.”

Eastern Car Service, he said, received more than 100 additional calls yesterday.