They also attacked Prime Minister Narendra Modi, saying decisions were being made outside Parliament when it is in session.
Digvijay Singh (Cong) raised objections soon after Minister of State for Finance Arjun Ram Meghwal tabled the notification on import duty cut.
"I want to bring to your notice that the notification reducing import duty from 10 per cent to zero per cent on wheat is primarily against the farmers.
"Wheat is being sold at much lower price in view of shortage of cash after demonetisation. It concerns the farmers. This will affect the farmers. This is a very serious issue. It is a very big conspiracy," Singh said.
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He was followed by JD-U leader Sharad Yadav, who also said the import duty reduction will adversely affect the farmers and hurt their interests.
"This is a serious issue. Government should think about it. This will destroy those sowing wheat in India," he said.
Chairman Hamid Ansari said, "A paper has been laid. You follow the processs. You raise the issue when the matter comes up for discussion."
He asked the members not to raise the issue as a procedure has been followed and they should raise their issues when the matter is taken up.
Meghwal earlier tabled a copy of a notification under which the government waived service tax on debit and credit card transactions of up to Rs 2,000.