The two-time defending Olympic champion Americans beat France 4-2, World Cup winner Japan got past Canada 2-1 and Brazil — the silver medallist in the last two Olympics — routed Cameroon 5-0 with two goals by five-time world player of the year Marta and one by Cristiane, who become the all-time top Olympic women's scorer with 11 goals.
Also, Britain edged New Zealand 1-0 in its first competitive game ever in women's football, giving the host the first victory at the London Games.
The football tournament started early because of its long match schedule. The men's competition will begin on Thursday.
The flag flap happened as North Korea prepared to play Colombia at Hampden Park. A North Korean player was introduced on the jumbo screen along with a picture of the South Korean flag, prompting the team to refuse to take the field and delaying the game start for more than an hour.
London Olympic organizers later apologized, saying that "clearly that is a mistake" and promising that "steps will be taken to ensure this does not happen again."
When the match finally started, North Korea came out pressing to win 2-0 with goals by Kim Song Hui in the 39th and 86th minutes.
In the first match in Glasgow, Scotland, the United States allowed two quick goals before rallying for its win over France. After Gaetane Thiney scored in the 12th and Marie-Laure Delie added another in the 14th, the U.S. came back with goals by Abby Wambach in the 19th, Alex Morgan in the 32nd and Carli Lloyd in the 56th. Morgan scored again in the 66th.
"It's not the game we wanted to play," U.S. defender and captain Christie Rampone said. "But we've got a lot of games ahead of us to try and get our rhythm back."
Wambach now has 139 international goals in her pursuit of Mia Hamm's record of 158.
Japan, which upset the U.S. to win the Women's World Cup last year, got goals from Nahomi Kawasumi in the 33rd and Aya Miyama in the 44th for its 2-1 win over Canada in Coventry. Melissa Tancredi pulled one back for Canada just 10 minutes after halftime but the Japanese held on.
"I think it was all about the defence today," Japan coach Norio Sasaki said. "We could run more than Canada and from the defence through to the attack everyone went for the ball."
In the later match in Coventry, Lotta Schelin scored twice to help Sweden defeat South Africa 4-1.
Brazil, runner-up to the U.S. in the last two Olympics and still in search of its first major title in women's football, had no problem getting past Cameroon in Cardiff, Wales.
It scored two early goals through Francielle in the seventh and Renata Costa in the 10th, then Marta converted a penalty kick in the 73rd, set up Cristiane's goal in the 80th and sealed the victory in the 88th.
Cristiane surpassed retired Germany star Birgit Prinz with her goal after coming off the bench in the second half.
Brazil has been close to winning a significant women's title several times. In addition to the Olympic defeats to the U.S., it lost the World Cup final to Germany in 2007, and last year was eliminated from the World Cup quarterfinals after allowing a last-minute equalizer to the Americans in extra time, eventually losing in a penalty shootout.
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