Saudi blogger Raef Badawi was flogged in public Friday near a mosque in the Red Sea city of Jeddah, receiving 50 lashes for "insulting Islam", witnesses said.
In September, a Saudi court upheld a sentence of 10 years in prison and 1,000 lashes for Badawi, and he is expected to have 20 weekly whipping sessions until his punishment is complete.
The United States, Amnesty International and Reporters Without Borders have denounced the flogging as a horrific form of punishment, and said Badawi was exercising his right to freedom of expression.
Witnesses said that Badawi, 30, was flogged after the weekly Friday prayers near Al-Jafali mosque as a crowd of worshippers looked on.
Badawi was driven to the site in a police car, and taken out of the vehicle as a government employee read out the charges against him to the crowd.
The blogger was made to stand with his back to onlookers as another man began flogging him, witnesses said, adding that Badawi did not make any sound or cry in pain.
The faithful who had emerged from noon prayers watched in silence and were ordered by security forces not to take any pictures on their mobile phones.
On the eve of the flogging, the US slammed it as an "inhumane punishment".
"We are greatly concerned about reports that human rights activist Raef Badawi will start facing the inhumane punishment of 1,000 lashes in addition to serving a 10-year sentence in prison for exercising his rights to freedom of expression and religion," State Department spokeswoman Jen Psaki said.
She said the US called on "Saudi authorities to cancel this brutal punishment" and to review his case.
Media watchdog Reporters Without Borders said the punishment was "barbaric" and noted its timing after Saudi Arabia condemned Wednesday's deadly attack on French magazine Charlie Hebdo.
"Although Saudi Arabia condemned yesterday's cowardly attack on the French satirical magazine Charlie Hebdo, it is now preparing to inflict the most barbaric punishment on a citizen who just used his freedom of expression and information," Reporters Without Borders program director Lucie Morillon said Thursday.
Badawi, who has been in jail since 2012, is a "prisoner of conscience", said London-based Amnesty International, demanding his release.
Badawi is the co-founder of the Saudi Liberal Network along with women's rights campaigner Suad al-Shammari, who was arrested last October and also accused of "insulting Islam".
"Flogging and other forms of corporal punishment are prohibited under international law, which prohibits torture and other cruel, inhuman or degrading treatment or punishment," said Amnesty's Philip Luther.