Arnaud scored in the 56th minute only to have Dominic Oduro equalize in the 71st as the Impact played to a 1-1 draw with the Chicago Fire in Montreal's first home game in MLS.
The Impact captain was mobbed in a corner of the pitch by his teammates as 58,912 fans raised a deafening cheer in the domed stadium Saturday.
"It was a special feeling, a special moment, not just for me but for the guys, the club and the fans," said Arnaud. "It would have been better if we got the three points, but it was a good moment."
The expansion Impact, who lost 2-0 in their first MLS game last week in Vancouver, earned a first point, but could have had more as they took the initiative from the opening whistle and created most of the scoring chances.
Had a long shot from left back Josh Gardner gone in instead of ringing off a goalpost in injury time, the festive day for the blue-and-white clad supporters in the seats would have been complete.
"It's kind of cool, but we have to continue to get better because if we get crowds like we had today, it's an unbelievable home field advantage for us," said Arnaud. "If we have that consistently, we're going to win some games here."
That won't be for a while, as the Impact play their next three games away from home starting next Saturday at Columbus. The next date at the Big O is April 7 against Toronto FC.
The Impact will play their first five regular season home games at Olympic Stadium until the expansion of the outdoor, 20,000-seat Saputo Stadium is completed in June.
After a scoreless first half in which forward Justin Braun was foiled by a big save on one chance and was called offside while scoring on another, the Impact finally ended 146 scoreless minutes as midfielder Felipe slipped a pass to the right wing to Sanna Nyassi.
The Gambian international stopped the ball, then lofted to the edge of the box for Arnaud to head over goalkeeper Paolo Tornaghi and into the net.
"It was a team goal," said Nyassi, who had been shifted in-game to the right wing by coach Jesse Marsch. "We knocked it from defence to midfield. Felipe got it to me and I took my time to get it to Davy. He was at the right spot at the right time."
Chicago, playing its season opener, struck back when Patrick Nyarko fed a pass to Sebastien Grazzini, who slipped the ball through for the onrushing Oduro to tip past Donovan Ricketts.
Chicago captain Logan Pause said his team did well to get a point in a difficult environment against a new team energized by the crowd.
"They have some talented players and, it being their opener and with that many fans, you knew they'd come out and attack and try to impose their will," said Pause. "But I thought we weathered the storm and hung in to half time at 0-0.
"I can't say enough about our response after giving up a goal. You really see the character of the team to fight back and claw and get a point."
Montreal was pleased at producing a better effort and a little more cohesion than they showed a week earlier in Vancouver. There was better play in the midfield from Felipe, Justin Mapp and Patrice Bernier and both Nyassi and Braun were more dangerous at the front.
"I thought we were better and more confident," said Arnaud. "We didn't give them much of anything, outside of scoring that goal.
"We know it can still get sharper. I don't think we've seen who we are and who we can be yet, but we'll get there."
The Impact broke the record attendance for a pro soccer match in Montreal of 58,542 set in 1981 by the defunct Montreal Manic for a playoff match, but fell short of the Canadian pro record of 60,342 by the Vancouver Whitecaps in 1983.
Marsch made no changes to the starting 11, but a cheer went up when newly acquired Italian striker Bernardo Corradi replaced Braun in the 77th minute. He set up Felipe for a chance, but the Brazilian midfielder was tripped up before he could shoot.