Protesters led by People's Movement Against Nuclear Energy (PMANE) convener S P Udayakumar were prevented from proceeding further by Rapid Action Force personnel and police, half a kilometre away from the plant, police said.
District Collector R Selvaraj and Sub-Collector Rohini Ramdoss and District Superintendent of Police V Bidari are holding talks with the protesters in a bid to persuade them to disperse.
More than 2,000 police personnel, including 400 RAF have been deployed near the site to prevent any untoward situation.
Prohibitory orders have been clamped in a seven km radius from the plant in view of the siege call given by PMANE, seen as a last ditch resort to prevent commissioning of the plant.
Regulatory authorities had recently given the go ahead for loading enriched uranium in the first of the two reactors.
The first unit of KNPP, an Indo-Russian joint venture, was scheduled for commissioning in December last, but was bogged down by delay due to protests against the project.