CLEVELAND -- Alfred Antenucci, head of the Ohio Building Trades Union, is hospitalized in Washington with an irregular heartbeat he suffered a few hours after he helped tackle the man accused of shooting President Reagan, labor officials in Cleveland said today.

Antenucci, 68, of Garfield Heights, Ohio, was in fair condition in the coronary care unit at Georgetown University Hospital.

Antenucci, head of the Ohio AFL-CIO Building Trades Union, and another Cleveland-area labor official, Frank J. McNamara, 62, were standing outside the Washington Hilton Hotel Monday afternoon, hoping to see the president when Reagan was shot.

Antenucci and McNamara, president of the Ohio State Building and Construction Trades Council, had just attended the AFL-CIO trade conference that Reagan addressed before he was shot.

They originally planned to go golfing, but stayed at the hotel because of rain. They tried to get inside the hotel to hear Reagan's speech, but the door was locked.

'We knew the speech was going to be finished soon, so we stood outside, waiting to glimpse Reagan,' McNamara said.

As they waited, Antenucci told his son, Dominic, by telephone Monday night, 'I saw a young man standing in front of me with his hands in his pockets.

'The next thing I knew, he had pulled a gun out of his pocket,' Antenucci said. 'I saw the gun fire two shots. My first reaction was to punch him. I punched him a couple times.'

Antenucci told his son he jumped on the alleged gunman, John W. Hinckley, 25, of Evergreen, Colo. In the fracas that followed, he said, he was struck by one of the Secret Service agents trying to disarm the man.

McNamara said he was standing near Antenucci when 'the president came out (and) this punk started shooting.'

'That's when we went after the guy,' McNamara said. 'Al grabbed him first. Al has more guts than anybody else.

'Then I started punching him (the gunman). I hit him so hard in the head I was left with blood on my knuckles,' he said.

McNamara said he wasn't afraid during the incident because 'things were happening too fast for me to be frightened.'

Both men were questioned by the FBI, and Antenucci became emotionally upset, complaining of an irregular heartbeat, his son said.

He was taken to Georgetown University Hospital by a Secret Service agent and admitted in the coronary care unit.

The two men headed a group of about 350 Ohio building trades people attending the three-day conference.