Brazil went top of Group E as they opened with a comfortable 5-0 victory over tournament newcomers Cameroon at the Millennium Stadium in Cardiff.
Early goals from Francielle and Renata Costa put the Samba Queens in control, before captain Marta with a brace and substitute Cristiane sealed victory.
USA, gold medallists at the Beijing 2008 Games, came from behind to edge out France 4-2 in a thrilling Group G contest, with Alex Morgan scoring twice.
Abby Wambach and Carli Lloyd were also on target for USA after Gaetane Thiney and Marie-Laure Delie had given France a two-goal lead.
Meanwhile, world champions Japan opened their account with a 2-1 win over Canada in Group F.
Japan established a two-goal advantage at the break, with the lively Nahomi Kawasumi and Aya Miyama both on target.
The Canadians restored their hopes with a strike from Melissa Tancredi after 55 minutes but could not find an equaliser.
Earlier, in the first action of the Games, Stephanie Houghton gave Host Nation Great Britain a 1-0 win over New Zealand in front of a passionate home crowd.
Houghton kept her nerve with a 25-yard free-kick in the 64th minute, curling her effort expertly into the bottom corner.
Sweden, meanwhile, showed their own credentials as they overcame Group F rivals South Africa 4-1.
Skipper Nilla Fischer, Lisa Dahlkvist and Lotta Schelin eased Sweden into a 3-0 lead before Portia Modise reduced the arrears with a spectacular long-range chip.
But Schelin restored the three-goal advantage, converting a near-post cross from Dahlkvist for her second goal.
DPR Korea ended an exciting day of action with a 2-0 win over Colombia - thanks to a brace from Song Hui Kim.