After having to wait two weeks since their opening-weekend defeat by South Africa, Scotland had the potentially all-important bonus point secured by halftime as they led 24-10 and added three more tries in a disjointed second half.
The win came at a cost, however, as captain and flanker Jamie Ritchie had to go off after being smashed in the head by the shoulder of Tonga winger Afusipa Taumoepeau late in the first half. Ritchie did not return and could face a period out due to concussion protocols.
The collision looked worthy of a red card but the bunker review decided there was some mitigation and Taumoepeau escaped with only 10 minutes in the sin-bin.
Ireland top the Pool B standings with 14 points, ahead of South Africa on 10. Scotland have five, with Tonga and Romania on zero. To have any chance of qualifying for the quarter-finals Scotland need to win their remaining games against Romania and Ireland, but even that might not be enough depending on the bonus point and points difference situation.
After their two-week layoff, Scotland were ahead after five minutes through George Turner after rolling a maul fully 25 metres.
Tonga looked off the pace in their 59-16 defeat by Ireland but were much more together on Sunday and went into a 10-7 lead after a brilliant run and offload by Charles Piutau slipped in the impressive Solomone Kata for their first try.
Scotland winger Duhan van der Merwe, used to physically dominating his rivals, found Kata something of an immovable object but he managed to avoid him in the 26th minute to score Scotland’s second try after some crisp work by Finn Russell and Blair Kinghorn. Another fizzing pass by Russell then sent Kyle Steyn in for the third.
Tonga’s indiscipline, particularly their wild tackling, was gifting Scotland penalties and points and then cost them a 34th-minute yellow card when Taumoepeau smashed a sickening blow into the face of Ritchie to send his head whiplashing horribly and ending his involvement for the day and possibly longer.
Rory Darge forced his way through to secure the bonus point with the last play of the half as Scotland led 24-10.
Taumoepeau returned to the fray and there was more good news for Tonga when 24-stone (152-kg) prop Ben Tameifuna bulldozed his way through five defenders for their second try.
Scotland struggled to build any attacking shape until the 54th minute when a great run by Van der Merwe –- including what must have been a satisfying shove to shift Kata - set up replacement scrumhalf George Horne to score in the corner and ease the tension with a two-score lead.
A try line siege then opened the way for a charging Kinghorn to score the sixth.
There was still time for a bunker red card for Tongan Number eight Vaea Fifita for smashing dangerously into Russell at a ruck and Scotland took advantage of the space with a carving run and score by Darcy Graham.