WASHINGTON—President Barack Obama will lay out sweeping changes to the immigration system in a speech Thursday night, offering new protections to millions of people in the country illegally and sparking a bitter fight with Republicans.

The executive action is expected to shield as many as five million illegal immigrants from deportation and offer them a chance for work permits, while overhauling the enforcement system and boosting border security, people briefed on the plan said. It would represent the most significant changes...