Late on Tuesday night, Grindeanu's cabinet approved a decree that would decriminalize "anti-graft" measures such as abuse of power, conflict of interest, and work negligence, effectively freeing a number of officials imprisoned for corruption.
It would have decriminalised abuse of power offences when sums of less than €44,000 (£38,000; $47,500) are involved.
"I am deeply concerned by the decree of the Romanian government", said U.S. Senator John McCain, a Republican from Arizona who is head of the Senate's Armed Services Committee. "It's my last wish to let this happen", Grindeanu said as quoted by the Romania-Insider newspaper.
European Commission vice president Frank Timmermans urged the government to "urgently reconsider" the decree, warning it could affect the EU funds Romania receives if it adopted.
"If we accept they can approve emergency decrees untransparently, then tomorrow they will adopt others and so on".
Earlier on Saturday the coalition government had hinted that it was considering backing down on the legislation.
About 80,000 people demonstrated in front of the government headquarters in Bucharest and thousands gathered in other cities, Reuters reports.
The prime minister said he did not want to "divide Romania" and that "Romania in this moment seems broken in two". At a time when the PSD leader, Liviu Dragnea, is seeking retrial for charges of election fraud, citizens have interpreted the reforms as a direct attempt by government officials to pardon themselves from criminal activity, both past and present.
But critics say the principal person to gain will be PSD leader Liviu Dragnea, 54, now on trial for alleged abuse of power involving 24,000 euros and who already barred from office for a previous conviction for voter fraud.
Earlier, Justice Minister Florin Iordache said he stood by the law, defying strong criticism from home and overseas and days of massive protests.
I feel I can't keep under control from the centre. the pressure from regional organisations that can bring one million people onto the streets of Bucharest.
President Klaus Iohannis followed Romania's top judicial watchdog in filing a legal challenge to the decree with the Constitutional Court.
Iordache, who had temporarily handed over his duties to a subordinate, told reporters Friday: "I take responsibility for this ordinance".
As horns blasted and marching bands swirled around him, Iuliam said corruption has always been institutionalized in Romania.
"If they can issue a law overnight they can do it again", she said.