Dude!

The audience at the Toronto Film Festival press screening of "Slumdog Millionaire" didn't know they were also going to get live entertainment Saturday.

There'd been lots of Oscar buzz about Danny ("Trainspotting") Boyle's flick, about a poor Mumbai guy who wins a girl and becomes a national hero by going on a game show. So the screening room was packed.

Soon after the lights went down, a source tells us, "a man in the audience started yelling, 'Don't touch me!' People looked around and shrugged. Ten minutes later, the voice yells again, 'I said don't touch me!'"

Again, people shrugged off the disturbance. But a few minutes later, says our source, "the guy stands up in the darkness and thwacks the guy behind him with a big festival binder. He hit him so hard everybody could hear it. Everyone freaked out and turned around."

The thwacker? New York Post film critic Lou Lumenick.

The thwackee? Esteemed Chicago Sun-Times film critic Roger Ebert.

After battling thyroid and salivary gland cancer for years, Ebert, 66, can no longer speak.

"Apparently, Roger was just trying to tap Lumenick on the shoulder to signal him that he couldn't see the movie," surmises our source. "He was trying to ask him to move over a bit."

Though Lumenick seemed surprised to see whom he had struck, he offered no apology, according to another source.

Lumenick didn't return a call and e-mail by deadline. Ebert's agent declined to comment.

Lumenick's lashing didn't prevent Ebert from enjoying the film. The critic walked out of the screening with two thumbs up — and handed Boyle a note saying he's bound to get an Oscar nod.