The former world and Olympic 10,000m gold medallist seemed to be struggling and stopped by the side of the road before walking away from the course.
The reason for his withdrawal is not yet fully clear.
The 41-year-old Ethiopian won the race last year in the fastest time ever recorded in Scotland for the 13.1-mile half-marathon distance.
His time of 61 minutes and nine seconds was also a world age-group record.
Scotland's Susan Partridge is bidding to retain the women's title against a quality field.
Around 30,000 participants of all ages started Scotland's biggest running event, which is televised live on the BBC.
Runners can choose between a 10,000m or half-marathon challenge through the heart of the city, starting in George Square and finishing on Glasgow Green.