Three people have been arrested as hundreds of protesters rally against Arizona's new immigration law despite a federal judge's last-minute decision to block the most controversial parts of the measure.

It was not immediately clear why the people were detained Thursday during the rally at the federal courthouse.

Demonstrators first marched from the state Capitol at dawn on Thursday, then held a prayer service at a local church.

Marchers are now heading to Maricopa County Sheriff Joe Arpaio's office and will then return to the Capitol. Activist Salvador Reza says some demonstrators have promised nonviolent civil disobedience, but he couldn't provide more details.