"I want to appeal to Congress to reconsider the decision. This is not proper," he told reporters here when asked to comment on Congress' decision.
Congress took the decision after its chief ministers, including B S Hooda of Haryana, were booed at rallies with the Prime Minister's functions recently.
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Singh said the Prime Minister himself had appealed to the public to remain calm in the functions but Congress still took the decision.
"Our Prime Minister believes in healthy democracy and believes that everyone should get the opportunity to speak in public function," he said.
Maharashtra Chief Minister Prithviraj Chavan boycotted the bhoomi-pujan of Metro Rail project in Nagpur done by Prime Minister Modi, and Hooda and Jharkhand Chief Minister Hemant Soren were heckled at functions attended by Modi.