Darren Clarke, who hasn't played much this year due to his wife's poor health, carded a bogey-free, 7-under-par 64. He was joined in third place by Raphael Jacquelin.
The rounds of 63 by Roe and Karlsson would have matched the course record at Club de Campo, but the round was played under lift, clean and place rules due to wet conditions.
Roe got his round going with a birdie on the par-5 fourth. He made it two straight as his birdie putt on the fifth fell into the cup. The three-time winner on the European Tour moved to 3 under with a birdie on the par-5 seventh.
The Englishman made the turn at minus-4 as he birdied the ninth. Roe parred the first three holes of the back nine. He then carded back-to-back birdies from the par-4 13th.
Roe got to 7 under with a birdie on the 16th. The 42-year-old closed his round with a birdie on the par-3 17th.
'I've never lost my card in 20 years and I'm very proud I can say that at the age of 42,' said Roe. 'So I won't be resting on my laurels. Last week playing my 500th tour event was a milestone and I was able to celebrate with my best finish for two years, but I came here intent on improving my position.'
Karlsson started his day on the back nine. He got on the board with a birdie on No. 11, then sank consecutive birdie tries from the 13th. The Swede also played his opening nine at 4 under as he birdied the 16th.
Around the turn, Karlsson sank a birdie putt on the first. He parred his next two holes before a making birdie on the fourth.
Karlsson then birdied the seventh, his third birdie on the three par-5s. He converted his final birdie try on the eighth to share the first-round lead.
'I did not expect to play this well today, specially with the rain, but I hit the ball solid, had my chances and putted well,' said Karlsson, who is looking for his first win since the 2002 Omega European Masters. 'Everything seemed to be working for me. However, golf is unpredictable, who knows what's going to happen tomorrow.'
Christian Cevaer shot a 5-under-par 66. He was joined in sixth place by Jose-Filipe Lima, Brett Rumford, Graeme Storm and Alastair Forsyth.
Jose Maria Olazabal and Gonzalo Fernandez-Castano, who won The KLM Open earlier this year, both ended at minus-4 and are joined there by nine other players.
Seve Ballesteros teed it up this week for the first time in two years as he has recently battled knee and back problems. The 50-time winner on the European Tour opened with a 6-over-par 77. He got off to a rough start as he played his first two holes in plus-3 and was unable to recover.