The mayor of Newark led a predawn protest outside an immigrant detention facility on Tuesday, trying to keep the jailhouse from becoming a critical part of the Trump administration’s ability to enact mass deportations.

The mayor, Ras J. Baraka, has been trying to stop the facility, which is expected to hold up to 1,000 migrants a day, from operating. In court, the city has argued that the detention center’s owner, GEO Group — one of America’s largest private prison companies — had begun operating without the required permits.

The early morning public pressure led to a tense, hourslong standoff. As activists held signs and chanted and the mayor waited in a misty rain, a GEO Group worker used a chain to lock the front gate.