Maoist guerrillas on Saturday fired upon a contingent of para-military Central Reserve Police Force in Chhattisgarh's Bijapur district, killing four troopers on a day when Chief Minister Raman Singh kicked off the ruling BJP's election campaign for Assembly elections."A squad of the CRPF's 168th Battalion, which was on an area-domination drive from a forested patch of Murdanda village to Timapur, came under intense firing from heavily- armed leftist insurgents," P. Sundar Raj, Deputy Inspector General (Naxal Operation), told IANS.The Maoists fired indiscriminately, followed by multiple IED blasts targeting CRPF jawans. The security forces retaliated, causing heavy damage to the rebels. However, four CRPF men lost their lives, while two others were critically wounded. They have been taken to the Basaguda field hospital for urgent medical attention.The attack took place late on Saturday and a heavy reinforcement has been rushed to the forested site to track down the insurgents.The attack also sent alarm bells in the political circle, as Maoists typically step up violence in the run-up to elections.Chief Minister Raman Singh addressed four public meetings in Bastar on Saturday amid heavy security arrangements.The state goes to polls in two phases -- on November 12 and November 20 -- to elect a 90-member Legislative Assembly. Polling in the violence-hit Bastar's all 12 assembly seats will be held in the first phase.