Those who may feel that Metrorail fares are too high, take note: There will be 16 hours of free rides Saturday, Sept. 19, courtesy of the AFL-CIO.

At a cost of $65,000 the labor group has leased the entire 37-mile network to help move thousands of people expected in Washington that day for Solidarity Day, a program of marches and speeches to protest the Reagan administration's cutbacks in social programs.

Fare gates at all 41 Metro stations will be locked open between 8 a.m. and midnight that day, allowing marchers and nonmarchers alike to pass through to trains.

"There is such a thing as a free ride," AFL-CIO spokesman Charlie Hughes said.

This will be the third time that Metrorail has sold itself to an outside party this way. In January 1977, the subways, then consisting only of six stations on the Red Line, were thrown open to the crowds attending Jimmy Carter's inauguration.

Last year, subway service was leased for two hours to a religious group called One Nation Under God to move the faithful from a rally at RFK Stadium to the Mall. For those two hours, service was free for all passengers between the two stations.

The AFL-CIO predicts that up to 100,000 people will show up for Solidarity Day. It turned to Metro weeks ago, discussed buying $1.20 passes en masse, then dismissed that as unwieldy in favor of the current plan.

Metro takes $45,000 in fares on a normal Saturday, according to spokesman Cody Pfanstiehl. The AFL-CIO agreed to kick in another $20,000 to cover the cost of extra trains and cars that will be needed between the hours of 8 a.m. and 8 p.m.

Metrobus service will not be free.