Armstrong’s best throw of 20.54 metres was well below his season’s best of 21.50, but good enough to top American Joe Kovacs (20.44) and Denmark’s Kim Juhl Christensen (20.02).
Armstrong, the defending Diamond League champion, leads the standings again this year. The Kamloops, B.C., native has reached the podium at all six of his outdoor meets this season, winning three of them. This was his first Diamond League victory of the season.
On Friday, Armstrong didn’t have to contend with world champion David Storl, Olympic champ Tomasz Majewski, world leader Reese Hoffa, or Americans Christian Cantwell and Ryan Whiting, who all skipped the meet. Those men are likely to be Armstrong’s toughest competition at the London Olympics.
Armstrong won his seventh consecutive Canadian shot put title last week in Calgary. In London, he'll will be trying for Canada's first Olympic throwing medal after missing the podium by a centimetre in 2008 in Beijing.
"I am right where I want to be in my training," Armstrong said. "Next competition is London (Crystal Palace) and then the Olympics."
Gay edges rival Gatlin
Tyson Gay beat Justin Gatlin at the line to win the 100 metres, avenging his loss to the 2004 Olympic champion at the U.S. trials last month for the London Games.
Gay dipped across the line in 9.99 seconds, .04 ahead of Gatlin. European champion Christophe Lemaitre of France was third in 10.08.
Gay won three gold medals at the 2007 world championships. But he has since been affected by various injuries and overtaken by world record holder Usain Bolt.
"I tried to be patient. I'm strong mentally and ready for challenges," Gay said. "The (U.S.) trials was a faster race, but this one was a better one for me technically."
Gay said the 100 will be wide open at the Olympics.
"There are a lot of people in the 100 metres now, it's open for the games," Gay said. "(Usain) Bolt, (Yohan) Blake, Gatlin ... it will be spectacular."
Gatlin, meanwhile, is confident he will improve significantly by then.
"Obviously, trials was better than here," Gatlin said. "Otherwise, the same — rain, wet track and a little cold. But we (also) had some jet lag. Now's the time to get some rest. I won the gold (in 2004) and the same thing could happen in London."
Lemaitre was disappointed with his run despite making up considerable ground with a late burst.
"I got out of the blocks behind everyone," Lemaitre said. "I was able to close the gap, but the bad start killed me. I will now run the 200 metres (at Crystal Palace), then it will be all training for the games."
In other events, Javier Culson of Puerto Rico in the men's 400 hurdles, Kenyan runner David Rudisha in the 800 and Dejen Gebremeskel of Ethiopia in the 5,000 set world-leading times this year.
Culson clocked 47.78 seconds and Rudisha won in 1 minute, 41.54 seconds. But Gebremeskel had more trouble, only pulling ahead of countryman Hagos Gebrehiwet in the last stretch to win in 12:46.81.
Kenenisa Bekele of Ethiopia, meanwhile, won't defend his 5,000 title at the Olympics after finishing ninth.
" I'm not that disappointed because I'll run in the 10,000," Bekele said. "I wish good luck to the three Ethiopians who will be in London."
Mariem Alaoui Selsouli of Morocco also set the world's fastest time this year to win the women's 1,500 in 3:56.15 seconds. Sally Pearson did likewise, the Australian blitzing the field in the 100 hurdles to run 12.40.
A heavy but brief shower dampened the Stade de France track before making way for bright sunshine.
Culson had little trouble adding to his victory at the Golden Gala meet in Rome in May.
He pulled ahead at the halfway stage and held off a late challenge from world champion David Greene (47.84). Felix Sanchez finished third, with Olympic bronze medallist Bershawn Jackson fourth.
Paul Kipsiele Koech led a 1-2-3 for Kenya in the men's 3,000 steeplechase.
Leevan Sands of the Bahamas comfortably won the triple jump, leaping a season-best 17.23 metres at his second attempt.
"The rain stopped at the right time for us," Sands said. "I wanted something around 17.20, so it all worked out just perfect."
European pole vault champion Renaud Lavillenie of France was not at his best but did enough to win with a vault of 5.77 metres.
In other women's events, Murielle Ahoure of the Ivory Coast finished strongly to win the 200 in 22.55. She held off American sprinters Bianca Knight (22.64) and Charonda Williams (22.70).
Amantle Montsho of Botswana won the 400 ahead of Jamaica's Novlene Williams-Mills, with Francena McCorory of the United States in third.
Tunisia's Habiba Ghribi overtook Kenya's Lidya Chepkurui near the line to win the 3,000 steeplechase.
"I gave everything in the home straight," Ghribi said.
Dani Samuels of Australia won the discus with a throw of 61.81 metres, with Croatia's Sandra Perkovic in second place.