Six people, including a policemen, died during protests in the area in January.
Earlier this week, the Communist-led state government promised to shift the proposed chemical industry hub out of Nandigram if locals continued to oppose it.
But Chief Minister Buddhadev Bhattacharya said the administration would have to "regain control" over the area and plans to send in riot police were announced.
Angry farmers along with political workers, belonging to the state's governing Communist party and the opposition Trinamul Congress, have dug up roads, burnt down wooden bridges and attacked government officials and policemen trying to enter Nandigram during the past two months.
The BBC's Subir Bhaumik in Calcutta says it is not clear why Wednesday's clash happened inspite of government assurances to the local farmers about their land.
On Wednesday morning, nearly 5,000 policemen set out to take control in Nandigram when protesting farmers prevented government and the police from entering the area.
Police officials say they ran into fierce resistance from thousands of farmers, both men and women, at the village of Bankaberia.
'Lawlessness'
Senior West Bengal official Prasad Ranjan Roy said the police fired tear gas shells to break up the protests, and then fired and charged through the protesting crowd when they came under attack.
"Nandigram has descended into lawlessness and no government can simply be inactive," he said.
Eyewitnesses say the local hospital is teeming with injured persons, many of them with bullet wounds.
The Trinamul Congress has called for a statewide strike on Friday to protest against the police firing.
Two allies of the Communist party have said the police action was "most unfortunate".
The issue of farm land acquisition has generated much emotion in West Bengal in the past few months.
The government's move to allot 1,000 acres of land to industrial giant, Tata Motors, to build a car factory in the Singur area in Hooghly district generated widespread protests.
State governments in India are acquiring large tracts of land to set up special economic zones (SEZs) to push up employment and earnings.
The federal government reckons that SEZs will bring in $13.5bn in investment and create 890,000 jobs by 2009 if the ambitious plan is allowed to proceed.
Critics say this is destined to become the biggest land grab in post-colonial India, given the lack of transparency and rampant corruption in government.
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