Drafted by the Ministry of Digital Affairs, the bill was meant to bring the EU’s Digital Services Act (DSA) into Polish law.
According to the government, it would have given authorities a faster way to restrict illegal material online, including hate speech, while protecting users' rights.
In a video statement, Nawrocki said the "spectre of the censor" had returned.
"It is not the role of an official to rule on whether a given piece of content was a crime. That is solely the competence of the justice system and independent judges."
He also accused the government of exploiting a series of deadly knife attacks in Polish schools to push the bill through.
Ministers had argued that the lack of national rules on blocking content made it harder for the authorities to act, but the president said such materials had been found and restricted under existing law in the following days.
"Emotional blackmail was used to push through rules that give the government what is essentially arbitrary control over what we can say, write or publish," he stressed.
Nawrocki described the bill as controversial and said that in normal circumstances it could be reviewed by the Constitutional Tribunal.
"But today there is no Tribunal," he argued, adding that a veto was therefore the only option.
He said he would not be subject to "blackmail" by the government, "especially if security and freedom are to be the hostages."
Digital Affairs Minister Krzysztof Gawkowski had urged the president to sign the bill, appealing to him "as a father."
"When we talk about children's safety online, behind the names of laws, regulations and political disputes lies an experience that every parent can imagine,” he wrote on social media.
He said the state could not allow its response to be one of helplessness.
Digital rights group Panoptykon Foundation had called for Poland to implement the DSA.
In April, its president, Katarzyna Szymielewicz, said that without new rules no Polish authority could effectively challenge a platform’s wrongful decision to remove lawful content, or its refusal to take down illegal posts, leaving court action as the only route.
She described that process as "neither fast, cheap nor effective."
Szymielewicz urged politicians to reach a compromise, warning that the continued delay "undermines the rights of Poles."
Panoptykon had proposed that the telecoms regulator (UKE) handle clear-cut cases such as online scams, while disputes over free speech go to the courts, with a faster procedure.
It is the second time the president has rejected legislation linked to the DSA.
The bill was one of two drawn up in response to his first veto, early this year.
The other, which allows the head of the regulator to be named Poland's digital services coordinator, was signed in late September and took effect on 6 November.
Nawrocki signed six other bills on Friday.
(ał)
Source: PAP, onet.pl, wp.pl