Six million riders of New York City's buses and subways face the threat of a crippling mass transit strike at midnight Monday.

A bus and subway walkout would create chaos on Manhattan's already clogged thoroughfares, making it nearly impossible for hundreds of thousands of mostly low-wage workers from outlying boroughs to reach their jobs in Manhattan Tuesday morning and deal a $150-million-a-day blow to the city's economy.

New Yorkers are gamely preparing as best they can for the strike, the first in 14 years. Hotels in midtown manhattan are booked solid. Big companies have hired buses from all over the East Coast to haul workers in from the suburbs and several firms even have hired excursion boats to ferry executives from Long Island, Brooklyn and Queens to their offices in midtown and Wall Street.

The area's transportation problems could be made even worse if Long Island Rail Road workers make good on their threat to strike Tuesday and suburban bus drivers do the same.

Late today, toll collectors on many of the bridges in and out of Manhattan said they will begin a work slowdown if they do not have a new contract by midnight Monday.

City officials have set strict rules for cars entering and leaving Manhattan, the city's central business borough. But New York City depends more heavily on its mass transit system than does any other city in the United States. All measures to deal with the loss of its extensive bus and subway systems at best will be palliatives.

Transportation talks here historically go down to the last minute before serious bargaining gets under way. The Metropolitan Transportation Authority finally put its first wage proposal on the table today. The proposed contract calls for three 3.5 percent increases over the next 34 months.

Transport Workers Union President John Lawe, whose union represents most of the 33,600 bus and subway workers, had demanded a 30 percent increase in wages over the next two years.

Last night, Columbia University law professor Walter Gellhorn, who chairs a three-member mediation panel, said the initial union reaction to the MTA proposal was, "not suprisingly," somewhat "negative."

But Gellhorn said that union officials indicated "that they were prepared to take a second look and lower their sights somewhat." Gellhorn, however, was not specific about the areas in which the union officials indicated they might be flexible.

Although money is the main issue, as it has been for nearly five years in this almost bankrupt metropolis, resolution of the central collective bargaining issues are complicated by outside factors.

The transportation authority is broke. Even if it granted no wage increases, it would run a $250 million deficit next year. An increase in the 50-cent bus and subway fare is inevitable.

Maintenance on buses and subways has been deferred for years to save money, to the point where serious delays and interruptions in service are commonplace and getting worse. If the subway system and the bus system are not to collapse, the authority must sharply increase its spending for maintenance.

Even the bare-bones offer MTA Chairman Richard Ravitch put on the table today would boost the authority's costs by $35 million this year, $70 million next year and $105 million in 1982.

Major politicians such as Mayor Edward Koch and Gov. Hugh Carey, whose presence could help speed a settlement, have stayed away from the talks. Neither politician wants to assume any responsibility for a settlement that will require an unpopular increase in fares, observers said.

The workers themselves, who settled for 6 percent annual wage increases three years ago, have been ravaged by inflation, and polls say the disgruntled union workers may not approve any proposed settlement that does not allow for hefty wage increases.

Lawe faces severe internal challenges within his union -- he won a reelection battle with three opponents last fall, but did not receive a majority of the votes -- and may have troubles bringing back a proposal that his members will approve. He barely controls a majority of the executive board of the union, which must give the initial nod to any agreement, and many of this allies on the board are not solid.

The MTA's Ravitch has said that any wage increase must be accompanied by productivity increases on the part of workers. Lawe has argued that the union has spent 40 years to get many of the work rules Ravitch wants to change and has vowed that it will not give up gains negotiated in previous contracts.