Hundreds of unionized drivers that work for the MBTA service, The RIDE, were on strike Monday over a health insurance dispute.
The 350 drivers for the para-transit service are contracted the firm Veterans Transportation.
Teamsters Local 25 said the members voted unanimously Sunday to reject the company's latest bid and go on strike immediately.
MBTA's The RIDE provides door-to-door service for people who cannot take public transportation because of a disability.
An MBTA spokesperson urged Veterans Transportation and the union to quickly reach an agreement. The MBTA is asking riders to postpone "non-essential trips" this week if possible, but said it will continue to provide transportation services "as best it is able."
"The MBTA's main priority is to provide transportation services for the T's most at-risk, vulnerable customers, and has developed a plan for RIDE service in the immediate time period," the spokesperson said.
With reporting from the WBUR Newsroom and The Associated Press