"I am astounded by the amount of weapons being smuggled from Ukraine to Belarus," Xinhua news agency quoted Lukashenko as saying at a ceremony marking the country's 30 years of independence following the collapse of the Soviet Union.
"This is why I've instructed the border guard to fully close the border with Ukraine," he said, adding that only civilians will be able to enter Belarus.
"We will not take all the others as prisoners," he said.
Without providing evidence, Lukashenko claimed that his security services had uncovered a foreign-backed terrorist sleeper cells plotting to oust him from power, the BBC reported.
"They have crossed the line. We cannot forgive them."
The President also alleged the weapons were being shipped to terrorist cells funded by Germany, Lithuania, Poland, Ukraine and the US.
Belarus shares a border with Ukraine in the south.
It borders Poland and Lithuania in the west, Latvia in the north, and Russia in the east.