ATHENS — Alexis Tsipras, the leftist political maverick who swept to power in Greece in a popular rebellion, was sworn in as the country’s new prime minister on Monday and immediately formed a new coalition government to charge into the task of reversing wrenching austerity policies and negotiating with European leaders to reduce Greece’s debt burden.

Dressed in a blue shirt, a dark blazer and no tie, he strode calmly into Maximos Mansion, the seat of the prime minister’s office, and took a civil oath rather than a religious one to assume the leadership of the nation.