The freight train was stationary when the speeding passenger train struck it at Walhar Railway Station in Rahim Yar Khan on Thursday, in the eastern Punjab province.
Jamil Ahmed, an official in the provincial government, said hospitals declared an emergency.
Twenty people were killed in the train crash and 74 were wounded, with some in critical condition, according to Javed Ahmed, a physician treating the wounded.
Prime Minister Imran Khan ordered authorities to provide the best possible medical care to the victims and said he was saddened by the news.
Khan ordered Railways Minister Sheikh Rashid Ahmad to "take emergency steps to counter decades of neglect of railway infrastructure" and ensure safety standards.
Authorities said Pakistan's army was also taking part in the rescue efforts.
Train accidents in Pakistan are often the result of poor railway infrastructure and official negligence.