Federal immigration agents descended on dozens of 7-Eleven convenience stores across the country before daybreak on Wednesday, arresting undocumented workers and demanding paperwork from managers, in what the Trump administration described as its largest enforcement operation against employers so far.

The sweeps of 98 stores in 17 states, from California to Florida, resulted in 21 arrests, according to United States Immigration and Customs Enforcement, which signaled intensified efforts against businesses that hire unauthorized workers.

“Today’s actions send a strong message to U.S. businesses that hire and employ an illegal work force: ICE will enforce the law, and if you are found to be breaking the law, you will be held accountable,” Thomas D. Homan, the acting director of the agency, said in a statement.

Mr. Homan, the nation’s top immigration-enforcement official, has promised more scrutiny of businesses that knowingly violate federal laws requiring employers to verify the identity and employment eligibility of their workers.