The pair of Beck taxi cabs were halted in the several feet of murky water that pooled in the tunnel near Front Street and Lower Simcoe Street.
Toronto firefighters waded into the knee-deep water, assisting several of the trapped passengers to exit the vehicles and walk to dry land.
"Our main worry was them not falling in that water," Toronto fire Capt. Murray Manson said.
One woman was lifted out of the cab and over a concrete barrier onto the sidewalk.
Manson described the underpass as a "dangerous" and a "problem" area most likely due to its proximity to Lake Ontario.
A photo was captured in the same Lower Simcoe tunnel of an abandoned two-door silver Ferrari after a record rainfall last month that caused extensive flooding and power outages across the Greater Toronto Area.
"People I think underestimate — they think they're going to get through it (and) it's not that deep," Manson said, adding water levels on Tuesday reached nearly a metre of water. "As soon as that happens your car is going to die and you're stuck in the middle."
There were also reports of a large puddle that had formed on the Museum Station subway platform but TTC service was not disrupted.