Watched by millions around the world, the hotly-anticipated clash at the Bernabeu was Portuguese Ronaldo's first game against the club where he made his name before joining Real for a world-record fee in 2009.
It also renewed the rivalry between Real coach Jose Mourinho and his United counterpart Sir Alex Ferguson as the La Liga champions seek the 10th European title that has eluded them since 2002 and the Premier League leaders chase a fourth triumph in Europe's elite club competition.
United withstood an early Real onslaught and drew first blood against the run of play in the 20th minute when Danny Welbeck nodded past Diego Lopez from a Wayne Rooney corner.
Ronaldo levelled 10 minutes later when he towered above Patrice Evra to send an Angel di Maria centre arrowing into the corner.
The result gave United a slight advantage for the return leg in Manchester on March 5, although both sides will regret squandering chances in the second half that could have eased their passage to the last eight.
Mourinho said he was confident his side could score more than one goal at Old Trafford next month.
"It's true they had two big chances in the second half but it was out of context because we were chasing the game and they were defending and waiting," the former Chelsea and Inter Milan manager told a news conference.
"Basically ,what I thought, everything's open, very open for the second leg. I think this will go to the last minute."
Ronaldo, top scorer in this season's Champions League with seven goals in seven appearances, barely celebrated his goal and spoke of the debt he owes United, where he was transformed from an inexperienced and fragile teenager into one of the world's most formidable forwards.
"It's a place where I played for six years and I have a lot of affection for the club as everyone knows," Ronaldo told Spanish television broadcaster Canal Plus.
"United are very strong in defence and they created some problems for us. But with the chances we had, we could have scored more goals," the 28-year-old added.
"Everything's open for the game at Old Trafford and I think we have the ability to score goals there and get through."
The two clubs were paired in Europe for the first time in almost a decade and as well as 2008 world player of the year Ronaldo the match showcased some of the game's top performers like United forwards Robin van Persie and Wayne Rooney and Real playmaker Mesut Ozil.
Real attacked the visitors' goal from the start, United goalkeeper David de Gea tipping a Fabio Coentrao effort on to a post in the sixth minute as a ragged United struggled to retain possession.
They had yet to have an attempt on goal when Welbeck spun away from Sergio Ramos and nodded inside the post for only his second goal in 28 matches.
Ronaldo, given a rousing ovation from both sets of fans before kick-off, went close following up on his own free-kick before his equaliser sent the home fans into raptures.
The Real faithful are hungry for Champions League success as the team's bid for a second straight La Liga title has floundered and they are stuck down in third 16 points behind leaders and arch rivals Barcelona.
Real were far more threatening going forward but nearly conceded a second in the 34th minute when Welbeck came close to poking a Van Persie centre past Diego Lopez.
United started the second half brightly but were soon on the back foot again as Real surged forward in search of a second.
De Gea, who joined United from Real's city rivals Atletico Madrid, again denied Coentrao on t
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