"When a Prime Minister reduces himself to ridiculing his predecessor-years his senior, he hurts the dignity of the parliament & the nation.
"He demeans his position and himself more than anyone else. Today's events were saddening and frankly; they were shameful," Gandhi tweeted.
Gandhi's remark came after Modi took a jibe at Singh in his reply to a debate in Rajya Sabha on the Motion of Thanks on President's Address, prompting the Congress to stage a walkout.
Modi had said, "So many scams occurred... We politicians have a lot to learn from Dr (Manmohan Singh) Sahab. So much happened, there is not a single blot on him. Dr Sahab is the only person who knows the art of bathing in a bathroom with a raincoat on."
"Within minutes of his speech, he attacked the former prime minister in the most unacceptable manner. He said that Manmohan Singh occupied various positions and one must learn from him how to take a shower wearing a rain coat. It was in extremely poor taste. It is unbecoming of a prime minister to use such language against a former PM. We are very very disappointed and angry (with) what the prime minister said. We expressed our protest by walking out (from the House)," senior Congress leader P Chidambaram said.
The former finance minister said that it was certainly unbecoming of anyone to say such "harsh and ugly" words against Singh.
Accusing Modi of "arrogance", another Congress leader, Kapil Sibal, said that the prime minister has "insulted" the House with his remarks and he must apologise.
"This is an insult of the House...we have never seen such arrogance. The prime minister should think that there is a stature to the post he holds and he does not know what words should be used...We will not tolerate this. We condemn the prime minister's remarks. He should apologise to the House for this," he said.
Targeting Modi, senior Congress leader Anand Sharma said, "He is behaving in an arrogant and insulting manner against the opposition. He also insulted the former late Prime Minister Indira Gandhi. He has dragged the debate to the lowest level."
Replying to a debate on the Motion of Thanks to the President's Address in the Rajya Sabha, Modi tore into the Congress and took on Singh who had described demonetisation as "organised loot" and "legalised plunder", saying "the art of bathing in a bathroom with a raincoat on" is known only to the former prime minister as there is "no blot on him" despite "all the scams".
This provoked an angry reaction from Congress members who staged a walkout in the midst of the reply by the prime minister.
"Modi is new to the House and he is here for only two-and-a-half years and he does not how to speak," Sibal said, adding that Singh had never attacked the prime minister in such language.
To a question on the former prime minister's 'plunder' and 'loot' remarks, Sibal said, "That is not the way the prime minister speaks. It was in the reference of ways of policy and that resulted in plunder and loot. It is not personal. Dr. Manmohan Singh never attacked the prime minister personally."
"Unless he (Modi) says that by mistake he said this and he apologises, how can you have a dialogue with this kind of language used against late Indira Gandhi and against Dr Manmohan Singh. You cannot get personal in a debate," he said.
Senior Congress leader Digvijay Singh said that never before has this kind of "derogatory" remarks been used against a former prime minister.
"I have never seen a sitting PM making such derogatory remarks against ex-PM. We for six years had Atal Bihari Vajpayee as the PM. He was always all praise for ex-PMs. This PM has been saying all that have happened in this country is because of him... This kind of a language we cannot accept," he said.
Another party leader, Renuka Chowdhury, said that she was "alarmed and sad" that this country was coming to this and "we walked out to keep our self-respect".
"I am alarmed, sad that this is what the country is coming to. You have a PM coming to Parliament and does not even keep that much respect for a fellow colleague and former PM. He has revealed himself to be such a small man... We cannot stoop to such levels so we walked out. Our self-respect is intact. Let the PM prove his after March 8," she said, referring to the last date of polling in the ongoing assembly election in five states.
Congress leader Ahmed Patel said that it does not "behove" of a prime minister. "I do not have words to condemn it," he said.