Reigning champions the USA stage an impressive comeback against France, as Brazil coast to victory over Cameroon.
Olympics: Women's football round-up - Brazil take charge
Team GB's Stephanie Houghton got the host nations' Olympics challenge under way with the only goal against New Zealand.
In front of a 24,549-strong crowd, the 24-year-old curled home a superb second-half free kick to give Hope Powell's side a win that puts them on track for a place in the last eight of the 12-team competition.
For the first 15 minutes, Team GB found themselves on the back foot against a New Zealand side that extended their hosts' defensive capabilities, with Hannah Wilkinson a particular danger.
That opening period was halfway through before Powell's side managed their first effort on goal, a curling Houghton free kick which was pushed away by Jenny Bindon.
It was the trigger for a more cohesive attacking display, with the supporters - large enough in number but swamped by the sheer size of the arena - right behind their team for all the debate about whether there should actually be a GB presence on the football pitch at these Games.
There was even a ripple of 'GB' around the stadium, although it came from the voices of school-age fans who will care little for the politics that overshadows a concept where reservations run deeper than Scottish duo Kim Little and Ifeoma Dieke opting not to sing God Save The Queen.
Arsenal fullback Alex Scott failed to make the most of a decent chance after she had skipped into the box and Anita Asante was also off target with a couple of headers.
Great Britain continued to create chances following the introduction of Ellen White at half-time.
Jill Scott seemed to duck under Houghton's cross as Bindon somehow pushed it behind, then Scott had a goalbound effort blocked by Ali Riley.
When White lacked the pace to race away from the New Zealand defence, allowing Riley to block, it added to the hosts' frustrations and brought a fear their superiority was not going to be rewarded.
To the Cardiff crowd's delight, however, in the 64th minute Houghton kept her nerve with a 25-yard free kick as she curled expertly into the bottom corner, with Bindon nowhere.
The cheers of delight were almost equalled by those of relief when a ridiculous mix-up between Scott and Dieke sent Sarah Gregorious clean through.
Gregorious panicked though, and rolled a pitiful attempt straight to Karen Bardsley.
Alex Morgan scored twice as the United States produced a stunning comeback from two goals down to beat France in their opening match.
The reigning Olympic champions looked in trouble at Glasgow's Hampden Park when Gaetane Thiney and Marie-Laure Delie handed France a 2-0 lead after 13 minutes.
However, Abby Wambach's header and a cool finish from Morgan brought the score level before half-time and further goals from Carli Lloyd and Morgan early in the second half gave the Americans a deserved victory.
These sides met at the semi-final stage of last year's World Cup, with the USA winning 3-1 on that occasion, but it appeared as if the French would gain revenge early on as they took advantage of sloppy American defending.
Thiney opened the scoring with a fierce effort from outside the area that Hope Solo should have done better with and two minutes later Delie doubled the advantage with a powerful drive, when after a number of ricochets around the area the ball fell perfectly at her feet.
The two-goal lead only lasted a further five minutes though as Megan Rapinoe's corner found Wambach at the back post from where she nodded in her 139th international goal.
Morgan made it 2-2 just after the half-hour mark as some awful defending allowed Solo's simple long ball to find the Seattle Sounders striker free to lift the ball over the advancing Sarah Bouhaddi.
Delie twice went close to restoring France's lead, while Morgan also had a big penalty appeal waved away.
The United States took control at the start of the second period and substitute Lloyd struck the crucial blow as she cut inside onto her left foot and fired a vicious shot into the corner from 20 yards after 56 minutes.
Another fine break from the Americans moments later sealed all three points as Tobin Heath was released down the left by Rapinoe and her cross found Morgan free at the back post to tap home.
Brazil went top of Group E as they opened their Olympic campaign in style with a comfortable 5-0 victory over tournament newcomers Cameroon at the Millennium Stadium.
Early goals from Francielle and Renato Costa put Brazil in control, before captain Marta with a brace and substitute Cristiane sealed victory.
The South Americans got off to the perfect start as they found themselves 2-0 up inside 10 minutes.
Firstly, midfielder Francielle fired a brilliant free kick beyond Cameroon keeper Annette Ngo Ndom to give the Beijing silver medallists the lead.
Defender Costa then doubled the advantage as she rose powerfully to head home from Francielle's corner.
Ester came close to adding a third goal for the Samba Queens but her shot from distance was tipped away by Ngo Ndom.
Cameroon had barely registered as an attacking threat during the first half, but their brightest moment saw some excellent trickery from Gabrielle Onguene, only for the striker to push her shot well wide.
Brazil's dominance continued after the break with Marta prodding wide after combining superbly with Thais Guedes.
But the skipper did add a third from the spot after 72 minutes before substitute Cristiane calmly rounded the keeper for the fourth.
Marta then rounded off the scoring by slotting into an empty net from Cristiane's pull back.
Sweden's women emphasised the gulf in class as they easily disposed of South Africa after a first-half scoring spree in their Olympic Games Group F clash at the City of Coventry Stadium.
Sweden demonstrated the gap between sides ranked fourth and 61st respectively in the FIFA rankings against a South African side competing in the tournament for the first time.
They dominated from the first whistle and inside the first five minutes Marie Hammarstrom and Caroline Seger both volleyed against the South African crossbar.
Sweden's skipper Nilla Fischer broke the deadlock after seven minutes although her shot from the edge of the box took a deflection off Refiloe Jane.
The woodwork again came to South Africa's aid when Hammarstrom struck a post from a centre by the dangerous Sofia Jakobsson.
But after 20 minutes Lisa Dahlkvist sidefooted home a Hammarstrom centre to double Sweden's lead.
Then 60 seconds later Lotta Schelin slipped the ball past South African keeper Roxanne Barker to make it 3-0.
Portia Modise reduced the arrears after 60 minutes with a spectacular long range chip over the head of Swedish keeper Hedvig Lindahl.
But three minutes later Schelin converted a near post cross from Dahlkvist for her second goal.
North Korea got their Olympic campaign off to a winning start over Colombia - but only after a delayed start which brought great embarrassment to London 2012 organising committee LOCOG.
Once the match got under way, North Korea proved too strong for their South American opponents with a double from Song Hui Kim giving them a 2-0 win.
Kim opened the scoring with a scrappy finish after 39 minutes but showed great composure four minutes from time to lift the ball over Sandra Sepulveda after the goalkeeper had spilled a routine cross.
North Korea can advance to the quarter-finals for the first time should they beat France on Saturday, with Colombia needing something against reigning Olympic champions the United States to avoid elimination in another double-header at Hampden.
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