City had fought back from 2-0 down to tie up the game at 2-2 before Van Persie's injury-time free-kick.
"It was a dagger in the heart - unbelievable," said Toure.
"We thought we would win it at 2-2, but United have great experience.
"It was a fantastic game for the fans though. That is why I like the Premier League. It is the toughest league in the world."