Supporters of PPP and MQM-P came face to face at the Safoora Goth polling station NA-235 in Karachi on Friday as the voting began in the country.
More than 26 million people would exercise their right of vote in the province, according to Election Commission of Pakistan.
Unofficial first results in the election are expected a few hours after voting closes at 5pm and a clear picture is likely to emerge early on Friday.
The caretaker government temporarily suspended mobile phone services on Thursday to strengthen security as voting began in the national election, the interior ministry said.