The British Open champion, rejuvenated following a fortnight's break, reeled off seven birdies for a blemish-free 64 after heavy morning downpours had forced organisers to suspend play for four hours.
Els, who teed off at the par-four 10th, picked up shots at the 13th, 14th, 15th and 18th holes to reach the turn in four-under-par 31.
He then birdied the first, sixth and seventh -- with a putt from 15 feet -- to consolidate his position at the top of the leaderboard as afternoon sunshine finally broke through at the par-71 Loch Lomond course.
Britain's David Howell, who also began at the 10th, carded a five-under-par 66 to climb into second place, with compatriot Phillip Price and Australians Peter O'Malley and Terry Price a further stroke back on 67.
However the lengthy rain delay in the morning meant half the field of 156 had not teed off by late afternoon, and 45 players were still out on the course when play was called off for the day.
First-round action will resume at 0630 GMT on Friday, with the second round scheduled to get underway 15 minutes later, officials said.
Els, 33, won four titles in his first five tournaments of the year. Although unable to maintain that form since, he is full of confidence for his British Open title defence at Royal St George's next week.
"I'm very pleased with that round today and I feel I'm quite sharp," said the world number two, who has not played tournament golf since tying for 11th at last month's Buick Classic in the United States.
"The majors are very important to me at this time of my career and I'm always up for them.
GOOD THINGS
"I made some bad swings today but, all in all, it was a good day for me. The greens are very soft and I made some good putts. I'm looking at good things to happen over the next couple of months."
The 28-year-old Howell, who mixed seven birdies with a double-bogey seven at the 560-yard 13th, was delighted with his round as he stayed on track to qualify for next week's British Open through a special money list.
"I played really well today and holed my fair share of putts," said the Englishman. "All in all, it was a good round.
"Driving on 13, I leaked it (the ball) on the right and it just went and went. Thankfully there were plenty of people to help find it but it was like a bog in there, real marshland.
"I wouldn't say it was easy out there today. After the storm, there was the added problem of picking up mud on the ball. The course is playing long and soft, it's like a dartboard."
Automatic British Open spots are on offer this week for the top seven and ties, who are not otherwise qualified, in the top 25 of a cumulative money list taken from the Volvo PGA Championship in May up to and including the Scottish Open.
Howell, at the moment, holds the final qualifying spot in that list.
"It's always a bit stressful and I think I've made it through two or three times that way," he said.
Defending champion Eduardo Romero of Argentina returned a one-over-par 72 while twice major winner John Daly carded a 74 that included a triple-bogey seven at the par-four 10th.
Of the late starters who managed to finish, world number eight Phil Mickelson carded a 76, seven-times European number one Colin Montgomerie a level-par 71 and twice U.S. Masters champion Jose Maria Olazabal a 73.
