The lawmakers came to the parliament building under heavy security as former Prime Minister Nawaz Sharif arrived, 13 years after his government was toppled in a military coup, Dawn.com reported.
"It is a good development that a peaceful democratic transition is taking place today," Sharif told reporters at the Islamabad airport. "There could not be a better way to change a government through ballot. I congratulate the nation over it."
Sharif is chief of the Pakistan Muslim League-Nawaz party, which is expected to join with opposition parties to form a coalition government led by Sharif.
If Sharif is elected Wednesday, he will begin his third term as prime minister.