Welsh welterweight Fred Evans and Scottish lightweight Josh Taylor made it three wins out of three for Great Britain's boxing team at the ExCel.
Evans, 21, did not have it all his own way against Algeria's Ilyas Abbadi but was too slick for his opponent, winning Sunday's bout 18-10.
Taylor, also 21, was good value for his 13-9 win over Robson Conceicao.
However, the Brazilian later claimed the judges had been influenced by a vociferous home crowd.
Irish welterweight Adam Nolan also won through with a victory over Ecuador's Carlos Sanchez.
The Wexford southpaw's win means Ireland also have three out of three boxers into the second round.
Despite winning European gold in 2011 and being world-ranked number two, Cardiff southpaw Evans was not seeded for the Olympic tournament.
But he made light of the oversight by outclassing his 19-year-old rival and
joining middleweight Anthony Ogogo in the second round.
"Amateur boxing is all about hitting and not getting hit and Fred Evans is great at that. He's a great counter boxer - has a star been born here? That was a fantastic introductory bout from Evans, it was really good stuff."
And while he appeared to get carried away with the home support at times and was hit by a couple of solid right hands, his straighter, more clinical shots ruled the day.
"I haven't experienced that sort of atmosphere before, with everyone screaming for me, and it put me on a high, it was one big buzz," said Evans.
"The only time I've seen a crowd like that before was at big pro fights.
"I started swinging at times but that was just a bit of ring-rust, and maybe the crowd getting me going a little bit and me getting a bit overexcited.
"But that should sharpen me up now for the next fight."
Evans faces number four seed Egidijus Kavaliauskas on Friday, seeking revenge for a defeat at last year's World Championships.
"When he beat me in the Worlds it was after my [Olympic] qualification fight so I was switched off completely," said Evans.
"But this time I'm looking at medals and he'll see a different fighter.
"He's good, he won bronze at the Worlds, but he suits me - he's a short, strong, come-forward fighter, nothing I haven't faced 100 times before."
Top seed and reigning welterweight world champion Taras Shelestyuk received a bye into the second round, as did two-time light-welterweight world champion Serik Sapiyev of Kazakhstan.
Cameron Hammond, bidding to become the first Australian boxer to win a gold medal, looked impressive in beating Moustapha Abdoulaye Hima of Nigeria 13-6.
Taylor and Conceicao engaged in a cagey first round before the Prestonpans southpaw settled into his rhythm and began to control the fight from range.
Interview - Team GB boxer Josh Taylor
Conceicao made it interesting in the third round as Taylor sought to protect his lead and, while it was close, the Scot was a deserved winner.
"The tactics were just to box and move, keep it long, and that's what I did," said Taylor, who now faces third seed and former world champion Domenico Valentino on Thursday.
"It felt very close but I thought I'd won it and was in control. I wanted to give the crowd a bit of a thrill but it's not about that, it's about winning the fight.
"I'll do exactly the same against Valentino, keep it long and box him."
Conceicao, meanwhile, thought it should have been him fighting the Italian.
"It was no way a fair result. I boxed very well and scored a lot of points," said the 21-year-old.
"I really thought the judges were influenced by the crowd. That shouldn't happen in a competition this big."
The favourite in the lightweight division is two-weight world champion Vasyl Lomachenko, one of the greatest amateur boxers of all time. The Ukrainian received a bye into the second round.
The fight of the night was between American lightweight Jose Ramirez and Frenchman Rachid Azzedine, Ramirez winning 21-20 after a ding-dong battle.
With welterweight Errol Spence also winning on Sunday, four out of four of the 12-strong United States team are now through to the second round.