Forty activists for civil rights for the blind demonstrated at National Airport last night to protest what they said are the discriminatory practices of USAir, but they failed in efforts to sneak supporters onto a departing USAir flight.

The protesters said they were angry about the airline's policy of denying seats near emergency exits to blind and incapacitated persons. They demonstrated for more than three hours in a cramped departure area and at USAir's main ticket counter because on Wednesday night Indianapolis lawyer Russell Anderson, who is blind, was removed from a USAir flight by Federal Aviation Administration police after refusing to move from an emergency exit seat.

Airline officials said they deny emergency row seats to the blind to avoid clogging plane escape routes.

Protesters said that if USAir again refused to give Anderson a seat next to an emergency exit on a flight to Indianapolis last night, dozens of them would board the DC9 to force the airline to give him the seat he wanted.

A tense 40 minute standoff in USAir's departure area followed, with blind activists threatening to storm the runway when ticket agents opened the door for ticketed passengers. There was some pushing and shoving.

But USAir and FAA officials defused the confrontation by delaying the flight, scheduled to take off at 7:25 p.m., and then directing ticketed passengers to four different doors in the departure area so they could board the plane, which left 50 minutes late.

The group, organized by the Baltimore-based National Federation of the Blind, then marched to USAir's counter in the main terminal and blocked it. But ticket agents directed passengers on the few remaining flights last evening to the departure gates, and there was no apparent disruption of ticket sales.

Marc Maurer, an organizer of the federation, said the demonstration would resume at National Airport this morning.

"Those people USAir sneaked around and got that plane off without dealing with the issue," Maurer said. "We're determined about this issue. It's not a matter of fad or indifference to us."

Airlines' policies vary with regard to the blind. Most forbid them from sitting in emergency exit rows. But the protesters said that such enforcement is arbitrary.

The federation said such rules single out the blind unnecessarily. The group, which threatened last year to practice civil disobedience against offending airlines, is one of a new breed that demands rights for the sightless. Some groups, like the American Council of the Blind, have condemned militant tactics.

The protesters defied airport officials' warnings that persons without tickets could not proceed through a metal detection device, and swarmed through the device.