Fresh examination for this paper will be held on May 10, an official announcement said late this evening.
Authorities deployed the Rapid Action Force and the Provincial Armed Constabulary to ease tension in Allahabad which witnessed violent protests.
The decision to cancel the first paper was taken after it leaked on Whatsapp messaging service just before the examination was to commence at 9.30 AM yesterday.
The meeting was attended by Chief Secretary Alok Ranjan, DG Police A K Jain and some other senior officers, besides UP PSC Chairman Anil Yadav, who was summoned from Allahabad, the headquarters of the Commission.
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"The Commission held its meeting this afternoon where, in view of a preliminary inquiry we had conducted into the allegations of question paper leak, it was decided to cancel the examinations for the first paper of the Preliminary exams held yesterday. Now the examinations would be held on May 10", UPPSC Secretary Rizwan-ur-Rahman said in Allahabad.
A senior STF official said the paper was leaked from Adarsh Bharti Vidyalaya College centre here allegedly by Vishal Mehta, Gyanendra and Jay Singh - all of whom have been arrested.
The three persons allegedly took photographs of the question paper an hour before the examination and it was circulated on Whatsapp.
Vishal is the son of Baldev Mehta, Manager of ABV College, which was made PCS exam centre for the first time.
Around 4.5 lakh candidates had appeared in 917 centres across the state, including 70,000 candidates in Lucknow who took the test in 148 centres.
For the second day, students protested at several places against the paper leak and demanded CBI probe and sacking of the Chairman, whose effigy they torched in Allahabad.