BUSINESS travelers have long been the bread and butter of the hotel industry in Manhattan, but competition for this segment of the traveling public is heating up as the city's overall hotel occupancy rate drops and more hotels go up.

One way some operators are staking their claim to a bigger share of the Manhattan hotel market is by providing larger and more sophisticated meeting spaces. Two of the most ambitious projects are nearing completion in midtown.

The New York Hilton and Towers Hotel, the largest convention hotel in New York, with 2,121 guest rooms and just under 100,000 square feet of ballroom, meeting and banquet space, will open a 36,000-square-foot expansion of its three-level convention area next month. The new space will be on the second and third floors of an office building under construction just west of the hotel.

In the other project, the developer Harry Macklowe is putting a 100,000-square-foot conference center in the mixed-use hotel and office building that his company is constructing on the block between 44th and 45th Streets near Broadway in the Times Square area. The hotel and conference center are to open in April.

Industry analysts expect the number of first-class transient rooms in Manhattan to grow by 10 percent by 1991. The current count is 45,000 rooms. Occupancy rates now are running at about 75 percent, a slippage from 78.7 percent two years ago.

Hotel industry analysts say the Hilton and Macklowe projects illustrate not only an increased demand for meeting space in Manhattan but the changing nature of the hotel industry as well. According to Thomas McConnell, a manager of the hotel industry group of Laventhol & Horwath, an accounting and consulting firm, most hotels view these meeting space components as drawing cards rather than profit centers.

''On a per-square-foot basis,'' Mr. McConnell said, ''the cost of building guest rooms or meeting or conference space comes out to be about the same - around $400 to $415 a foot in the city. The extra expense comes in the operation of these facilities.''

Stephen W. Brener, a Manhattan-based hotel consultant, said two factors were behind the lower occupancy rates: rooms added in recent years and budgetary belt-tightening in the corporate world.

''The individual business traveler is still the most valuable customer for most hotels at the upper end,'' he said. ''They represent the largest segment of the market and they pay the highest rates. But the business traveler is harder to get now because companies are economizing by cutting back on travel expenses.''

John F. Power, general manager of the Hilton hotel, said: ''This addition will significantly increase our ability to host groups that we could not accommodate before. Not only will it give us more room, but the new space will have state-of-the-art technology that will allow both those gathering for meetings as well as exhibitors maximum flexibility and advanced audio-visual capabilities.''

The Hilton's new space, which the hotel is calling Americas Hall, is in the 35-story office building being built next door by Edward J. Minskoff and A. Alfred Taubman. They bought the site a few years ago from Hilton Hotels and the Prudential Insurance Company after Hilton decided not to go forward with a plan to build an addition. As part of the deal, the developers agreed to provide specially built space with high ceilings and reinforced floors for the hotel's convention space expansion.

A glass-enclosed bridge will link the new convention halls with the existing meeting and ballroom space on the hotel's second and third floors. Each floor has 18,000 square feet of open space that has been fitted with power lines, 132 telephone jacks, 10 cable televison outlets and the technology needed for satellite or in-house video transmissions. Mr. Power said the new space could accommodate up to four different groups at one time.

With its new venture, the Macklowe organization is seeking to attract a different segment of the meeting market. Rather than trade shows and large conventions, the developer hopes the conference center will appeal to corporate clients that want to hold small meetings but do not want to leave town to do it.

The Macklowe Conference Center will occupy five floors in the 638-room hotel and will incorporate the adjacent Hudson Theater, a landmark auditorium that Mr. Macklowe bought for the air rights that allowed him to raise his new building to 52 stories. The 1,100-seat theater, built in 1904, will be be restored by the developer and will serve as an auditorium for the conference center. It will also be rented for independent events.

The main entrance to the conference center will be on 45th Street, while the lobby for the hotel will be on the 44th Street side of the building. The conference space will include 33 meeting rooms featuring video and rear-screen projection equipment.

photo of Hilton Hotel (NYT/John Sotomayor)