The accused, identified as Aslem, was nabbed in Kairana area on the basis of a video clip of the firing incident that took place last week, Circle Officer Nishank Sharma said.
This is the fifth arrest made in the case so far.
As many as 200 unidentified persons have been booked under section 147 (rioting), 148 (Rioting, armed with deadly weapon), 149 and 143 (unlawful assembly), and 188 (Disobedience to order duly promulgated by public servant) of IPC in this connection yesterday, SP Vijay Bhushan said.
Uttar Pradesh Chief Minister Akhilesh Yadav has announced a financial assistance of Rs 5 lakh to the kin of the boy killed in firing, an official spokesman said here.
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The CM said that the guilty will not be spared and strict action will be taken against them.
Four other accused had been arrested yesterday in this connection on the basis of the clip.
The SP workers were celebrating the victory of party candidate Nafisa in the local body polls from the town when eight-year-old Sami, who was passing by Kairana area in a rickshaw, was killed in the firing.
Hasan had refuted allegations that he or his aides had attacked the two journalists and said it was an attempt by the opposition to tarnish the image of the ruling party.
Hasan has been served a show-cause notice by his party, while the victorious block pramukh was expelled.
The incident has invited sharp criticism with the BJP terming the Akhilesh Yadav government as a 'Mughal sultanate' that has issued license to its "goons" to do what they want.