ISLAMABAD—Pakistan’s parliament elected opposition lawmaker Shahbaz Sharif as the new prime minister Monday, following a week of political turmoil that led to the weekend ouster of Premier Imran Khan. Sharif, the brother of disgraced former Prime Minister Nawaz Sharif, won with 174 votes after more than 100 lawmakers from Khan’s Pakistan Tehreek-e-Insaf, or Pakistan Justice Party, resigned and walked out of the National Assembly in protest. Those 174 votes—two more than the required simple majority—are enough to pass laws in the 342-seat assembly. If Khan’s followers take to the streets, as he has vowed, it could create more pressure on lawmakers and deepen the crisis. Khan was ousted early Sunday. He lost a no-confidence vote after being deserted by his party allies and a key coalition partner. Khan rallied hundreds of thousands of supporters Sunday night to protest his ouster, describing the new leadership as an “imposed government,” charging they …