"It remains the single biggest threat to our stability and to our progress," he told the inaugural session of the 15th summit of the South Asian Association for Regional Co-operation (SAARC).
"We cannot afford to lose the battle against the ideologies of hatred, fanaticism and against all those who seek to destroy our social fabric," he warned.
He said terrorists and extremists know no borders and that the recent attack on the Indian Embassy in Kabul and the serial blasts in Bangalore and Ahmedabad in the last few days are "gruesome reminders of the barbarity that still finds a place" in South Asia.
"We must act jointly and with determination to fight this scourge," Singh said. "We must defend the values of pluralism, peaceful coexistence and the rule of law."
Other leaders also mentioned terrorism in their speeches at the opening of the summit.
Hamid Karzai, president o