"We have arrested R Nongrum, general secretary of Khasi Students' Union Lawsohtun unit and S Nongkynrih a senior member of the organisation, district police chief M Kharkrang told PTI here.
The two were arrested when they were en route here in a taxi for tomorrow's rally, he said.
Based on their interrogation, two other KSU members, P Lyngkhoi and Johny Sohkhlet, both of Lawsohtun area, were arrested in neighbouring West Khasi Hills district, he said.
Businessman Vikas Nandwal was set on fire after petrol was poured on him on in his store on October 9.
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Nandwal who suffered 60 per cent burns died in a New Delhi hospital October 26, with KSU members Wankerlang Jyrwa and Badonkupar Nongbri, arrested and charged with the murder.
The 74-year-old owner of a textile shop, Vidya Devi Chokhani, died after a petrol bomb was hurled at her shop.
Petrol bombs were also hurled at a CRPF camp here in September.
Leaders and members of pressure groups led by Khasi Students Union and Federation of Khasi Jaintia and Garo People were in hiding, ahead of the rally.
Security in and around the state capital was tightened and vehicles were being searched, Kharkrang added.