Joe Biden has described as a "dangerous precedent" a Supreme Court ruling giving former President Donald Trump partial immunity from criminal prosecution.
The current US president said the judgement undermined the "rule of law" and was "a terrible disservice" to Americans.
Earlier, Trump hailed the court's decision as a "big win" for democracy.
The justices found on Monday that a president had immunity for "official acts" but was not immune for "unofficial acts", and referred the matter back to a trial judge.
The judgement will further delay the criminal case against Trump for allegedly trying to subvert the 2020 election result that gave victory to Mr Biden.
The trial judge must now determine which actions were carried out in Trump's capacity as president, which could take months. Any trial is unlikely to start before November's presidential election.
In a televised statement late on Monday, President Biden said: "This nation was founded on the principle that there are no kings in America. Each of us is equal before the law. No one, no one is above the law. Not even the president of the United States.
"Today’s [court] decision almost certainly means that there are virtually no limits on what a president may do.
"The man who sent that mob to the US Capitol is facing potential criminal conviction for what happened that day. The American people deserve to have an answer in the courts before the upcoming election."
Mr Biden was referring to Trump being on trial for his alleged role in stirring up the riot.
"Now, because of today's [court] decision, that is highly, highly unlikely," Mr Biden said.
Following the Supreme Court's decision, the trial judge must now determine which actions were carried out in Trump's capacity as president, which could take months. Any trial is unlikely to start before the 5 November election.
This is a huge boost for Donald Trump - a "big win" as he put it on his social media platform Truth Social.
The Supreme Court ruled that all former presidents have partial immunity from criminal prosecution - total immunity applies to acts carried out as part of the president's official duties, but "unofficial acts," in a private capacity, are not covered.
A lower court judge will now have to decide which aspects of the president's behaviour are relevant to the criminal prosecution where he is accused of trying to overthrow the result of the 2020 election.
The majority opinion by the Supreme Court deemed his interactions with the Department of Justice (DoJ) immune from prosecution.
Trump is accused in his indictment of pressuring DoJ officials to launch investigations into voter fraud despite a lack of evidence.
The three liberal justices on the Supreme Court strongly dissented from Monday's decision. Justice Sonia Sotomayor said: "The president is now a king above the law."
Democratic Congresswomen Judy Chu said the fallout from the court's decision would be far-reaching.
“If a president says in any official capacity that they want to do something that we would consider to be improper and criminal, he could be immune from the actions that he takes," she said.
The court's six-three ruling will significantly delay any trial - if it is ever to go ahead - until well after the November election.
The ruling will also apply to the other outstanding criminal prosecutions facing Donald Trump, relating to the top secret documents found at his home in Florida, and the case in Georgia where he is accused of conspiring to overturn his narrow election defeat in the state.
According to OceanNewsUK's news partner CBS, Trump's legal team is seeking to overturn his conviction in New York, where he was found guilty in May of 34 counts of falsifying business records related to concealing an alleged sexual encounter with former adult-film star Stormy Daniels.
Trump's legal team have sent the judge in the case a letter related to this effort and cited the Supreme Court's opinion, CBS reported, citing a source familiar with the matter. The letter in the case has not yet been made public.
