Cinemas, hotels, public parks and even army bases across the Philippines fell silent on Sunday as Pacquiao, the only boxer to win world titles in eight weight divisions, tasted his second straight defeat this year.
"I'm so shocked, I can't believe it when Manny was ahead on points," barber Pedro Varela told Reuters after watching the fight at a cinema in a Manila shopping mall.
"It was a good fight, Manny gave it all. He was impressive, It was one lucky punch from Marquez."
The 39-year-old Marquez, who had lost twice and drawn once in their three previous meetings, sent Pacquiao crashing to the canvas with a stinging right hand with a second left in the sixth round of their non-title welterweight bout at the MGM Grand Garden Arena in Las Vegas.
The Filipino southpaw remained motionless for several minutes before finally getting up, smiling at Marquez and then shaking his opponent's hand in middle of the ring.
Social media sites were abuzz after the fight, with fans praising both fighters while there was also outpourings of disbelief that Pacquiao lost.
The army said Pacquiao, an elected congressman and a lieutenant-colonel in the reserve force, was still "an icon of perseverance, excellence and dedication not only to the soldiers but to millions of Filipinos".
"Pacquiao did his best but Marquez turned out to be the better boxer," military spokesman Colonel Arnulfo Burgos said in a statement.
Some Filipinos said Pacquiao might retire after the loss.
"I think that's the end of his career," state worker Miguel Manalo said.
Manila's streets were deserted during the fight as Filipinos tuned into see the action. Police authorities have said crime rates drop every time Pacquiao enters the ring.
(Reporting By Manuel Mogato and Roland Ng; Editing by Peter Rutherford)