"If BJP comes to power, we'll institute an inquiry to find out how so many people of a particular community could enter the country illegally and settle down," Singh said during a rally at Karimganj in Barak Valley.
"If, however, anybody comes with proper passport, visa or work permit, we have no problem with that but we cannot accept if they do not go back and settle down here," the BJP chief said.
"Everybody knows that there has been a huge influx of people of a particular community from Bangladesh during the last four decades and certain political parties have patronised them for vote-bank politics," he said.
"In the case of Hindus being harassed in Bangladesh and forced to flee that country, we cannot treat them as refugees, but must be sympathetic to their cause," he said.
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"BJP does not believe in distinction between different religions or on division of the country on the basis of religion as was done during the time of Independence," he said.
"Earlier, before BJP came to power in 1998, India was considered a weak country and this was a matter that always hurt Vajpayeeji. After coming to power, he decided that India will carry out nuclear tests and though many countries opposed it, he went on with the tests," he said.
"Powerful countries threatened to stop economic aid and imposed sanctions, but we finally emerged strong and no country considers us weak now," Singh said.
During the six-year NDA rule, the government was successful in containing price rise but "whenever Congress comes to power, there is always unprecedented price rise. This is mainly due to wrong economic policies, planning and above all corruption", he said.