WASHINGTON (AP) _ President Reagan today hailed the 200-year-old Constitution as a ''blueprint for freedom'' and then led thousands of school children, federal workers and tourists in reciting the Pledge of Allegiance.

Shortly afterward, 100,000 red, white and blue balloons were released to end a 3 1/2 -hour tribute to the bicentennial of the Constitution.

''Times have changed, but the basic premise of the Constitution hasn't changed. It's still our blueprint for freedom,'' Reagan told the crowd that filled the west lawn of the U.S. Capitol.

''For over 200 years we have lived with freedom under law, and perhaps we have become complacent about it. We should never forget how rare and precious freedom is,'' Reagan said.

Senate Majority Leader Robert Byrd, D-W.Va., said Americans may sometimes ''take it for granted that the president and the Congress share the power.''

''But even today,'' Byrd said, ''we must check and balance each other. From our experience, we know that when one branch of the government gets too much power, we lose control of our democracy.

''Usually, America has gotten hurt as a result.''

House Speaker Jim Wright, D-Texas, urged the audience to ''resolve that we shall expand the reach of liberty and justice in our own time to embrace those Americans now denied the reality of its promise.

''Let us resolve,'' he said, ''to preserve its benefits intact for future generations, and to honor its spirit in our lives by truly respecting and protecting the rights of those even with whom we disagree.''

The first speaker, Education Secretary William Bennett, told the crowd, ''The Constitution is a double-edged sword. It is our protection but we are also its protectors.

''Two hundred years after its ratification ... the Constitution remains uniquely worthy of our allegiance,'' Bennett added. ''Happy birthday, Constitution.''

The tribute was held a day before the anniversary of the Sept. 17, 1787, signing of the Constitution.

John Peschong, a spokesman for the Commission on the Bicentennial of the U.S. Constitution, said organizers didn't want to take away from the celebration on the anniversary date in Philadelphia, where the Founding Fathers conceived and wrote the Constitution.

''But we thought something needed to be done in the nation's capital because it is the home of the Constitution - it is kept in the National Archives - and because we wanted to involve the three branches of government,'' Peschong said.

''By holding our celebration a day early, Americans can reflect on what the Constitution means to them,'' he added. ''The former chief justice has said he wants the day to be a history and civics lesson, and this way, people will understand better what they are celebrating on Thursday.''

The commission has encouraged schools across the nation and on military bases in foreign countries to conduct daylong lessons on the Constitution and has urged them to tune in their television sets today so students can join Reagan in reciting the pledge and Burger in reading the preamble.

Fourteen-year-old Stephanie Pett, a student at Independence Middle School in Bethel, Pa., spoke about ''what the Constitution means to America's youth and their future.'' She was the winner of the Scripps-Howard National Spelling Bee. She was followed by Damien Atkins, 17, of Banneker High School in Washington, who introduced Reagan.

Two other Washington students, Brian Morris and Tyese Wright of Kendall Demonstration Elementary School, used sign language to accompany Reagan as he recited the pledge.

Congress was in session today, but took time off for the ceremony.

Federal workers are getting a break today too. Their usual one-hour lunch is being extended by 90 minutes so they can attend the ceremonies.

Peschong said the commission purposely steered away from planning a flashy tribute to the Constitution. ''We're trying to keep this a very historic occasion,'' he said.