"It is unfortunate that we get to know about prime minister's visit through a tweet... India and Pakistan relations are not so good as yet that he stops over there on his way back from another country," Congress spokesperson Ajoy Kumar said.
"The parliament session got over just a few days back, why was the parliament and the country left in dark. Why didn't the prime minister take the country and parliament in confidence," he questioned.
Congress leader Manish Tewari called it an "adventure" by the Indian prime minister.
"This adventure of prime minister will have serious impact on national security, there were no changes in relationship (with Pakistan) by NSA talks, or Heart of Asia conference. Eventually prime minister needs to answer what has changed in ground situation," Tewari said.
In a sudden development on Friday, Modi said he will meet his Pakistani counterpart Nawaz Sharif in Lahore in the afternoon on his way back to New Delhi from Afghanistan.
Modi also wished Sharif on his birthday.
Friday also marks the 139th birth anniversary of Muhammad Ali Jinnah.