As he often does, Van Morrison looked uncomfortable on stage when he first appeared at the Merriweather Post Pavilion last night. With his black jacket buttoned taut across his round belly, his balding head framed by white muttonchops and his back ramrod-straight, he looked more like a defrocked Irish priest than the greatest male singer in the English language today. Gradually, though, after a few perfunctory numbers, he seemed to relax, close his eyes and tap into the improvisational soul singing and Celtic mysticism that have made him legendary.

You could hear him unwind a bit on the "meet me down by the river" improv section of "You Know What They're Writing About"; you could hear him loosen up even more on the stately Irish folk standard "Ragland Road," growling like a lion and singing a scat duet with Georgie Fame. When he picked up his black Stratocaster guitar on "Northern Muse (Solid Ground)," his throat opened up fully and his most instinctive music, his most elemental emotions began to pour out into the microphone unimpeded, climaxing in a jaunty snatch of "Auld Lang Syne."

On the eighth song, "Sweet Thing" from 1968's "Astral Weeks" album, the 45-year-old Belfast singer and the jampacked crowd entered that transcendent realm that few singers ever taste and few audiences ever experience. As London's legendary Georgie Fame & the Blue Flames established the sinuous groove, Morrison quickly departed the usual melody and began to improvise variations on the song, first in intimate purring whispers and then in explosive soul shouts. On and on the open-ended arrangement went, as Morrison sustained the aura of suspense and intensity for more than 10 minutes.

The rest of the show was one dizzying triumph after another. He roared through "Vanlose Stairway" and milked his Sinatra-like ballad "Have I Told You Lately That I Love You" of every possible nuance. "Common One" was another 10-minute-plus tour de force, with Morrison and saxophonist Richie Buckley engaged in a feverish call-and-response duet.

The first encore featured "Moondance" and a spellbinding reading of "In the Garden." The second encore featured a boisterous, shouted "Brown-Eyed Girl" and an extended, dramatic version of "Caravan." The third encore served up a Morrison-Fame duet on "Buena Sera Senorita" and a comic sing-along version of "Gloria." The fourth encore was an irreverent treatment of Stephen Sondheim's "Send in the Clowns"; the summer was officially over and so was the season's best concert.