The incident occurred in Daraza Sharif area of Khairpur district, senior police officer Peer Muhammad Shah said.
"Eleven people were killed in an armed clash between the groups," he said.
Shah said that the situation was tense and army was called to avoid further violence.
There were also reports to clashes and quarrels between various groups in different parts of Punjab and Sindh where elections were held in 20 district in the first stage of the three-phase elections.
Despite violence polling went ahead and was completed in majority of places at 5.30 pm and counting of votes had started.
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The elections are being held under a security umbrella of Pakistan army which was asked by the election commission to keep peace on the polling day as several polling stations were marked as sensitive.
The 12 districts in Punjab where polls were held include Lahore, Faisalabad, Gujrat, Chakwal, Bhakkar, Nankana Sahib, Kasur, Pakpattan, Okara, Lodhran, Vehari and Bahawalpur.
Eight districts of Sindh included Shikarpur, Larkana, Jacobabad, Kashmore, Qambar, Sukkur, Khairpur and Ghotki.
In Punjab, the ruling Pakistan Muslim League-Nawaz (PML-N) is expected to get majority of seats despite a tough challenge by Pakistan Tehreek-e-Insaf of Imran Khan.
The media was giving provisional results after, the Lahore High Court (LHC) lifted ban imposed by election commission to air unofficial results.
Reports said that dozens of candidates were already elected unopposed both in Punjab and Sindh.