Rudy Giuliani on Thursday slammed Michael Cohen and special counsel Robert MuellerRobert (Bob) Swan MuellerSasse: US should applaud choice of Mueller to lead Russia probe MORE in the wake of a new plea agreement between the two parties, saying the timing of the announcement was meant to harm President TrumpDonald John TrumpTrump calls Sri Lankan prime minister following church bombings Ex-Trump lawyer: Mueller knew Trump had to call investigation a 'witch hunt' for 'political reasons' The biggest challenge from the Mueller Report depends on the vigilance of everyone MORE.

Cohen on Thursday agreed to cooperate in the special counsel’s investigation and pleaded guilty to one count of making false statements to Congress related to his testimony about communications with Russia during the 2016 presidential race.

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"It's no surprise that Cohen lied to Congress. He's a proven liar who is doing everything he can to get out of a long-term prison sentence for serious crimes of bank and tax fraud that had nothing to do with the Trump Organization," Giuliani, who represents Trump in the Russia probe, said in a statement.

“It is important to understand that documents that the Special Counsel’s Office is using to show that Cohen lied to Congress were voluntarily disclosed by the Trump Organization because there was nothing to hide,” the former New York City mayor added.

Giuliani also said that Mueller timed Cohen's guilty plea to coincide with Trump's departure for the Group of 20 (G-20) summit in Argentina. He likened Thursday's announcement to when Mueller announced charges against a dozen Russian military officers days before the president met with Russian President Vladimir Putin in Finland.

Cohen, the president's former personal lawyer, said he was untruthful about his involvement in plans to build a Trump property in Russia in order to remain loyal to the president and consistent with his “political message.”

Cohen previously told Congress that negotiations to build a Trump Tower in Moscow ended in January 2016, but emails showed that he continued to talk with a Russian developer about the project into the summer of 2016 and kept Trump informed of the talks amid the presidential campaign.

Shortly after news of the guilty plea broke, Trump slammed his former associate as a “weak person” and accused him of “lying” in order to get a lesser sentence.

"He's a weak person and not a very smart person," Trump told reporters before leaving for Argentina. "What he’s trying to do is … he’s got himself a big prison sentence and he’s trying to get a much lesser prison sentence by making up a story."

The president offered a similar condemnation of Cohen after his former attorney pleaded guilty earlier this year to violating campaign finance laws, which he said was at Trump's direction.

Those charges stemmed from a $130,000 nondisclosure payment he gave to Stormy Daniels to silence the adult-film star, who claims to have had an affair with Trump more than a decade ago.

Prior to his first guilty plea, Giuliani had defended Cohen as an "honorable lawyer" and said he had no concerns about Cohen cooperating with prosecutors.

The former New York City mayor later said he did not know Cohen very well when he defended him.

Giuliani and Trump have both been fiercely critical of the Mueller investigation into Russian interference in the 2016 election, decrying it as a "witch hunt" and insisting the president has done nothing wrong. Trump's legal team submitted written answers to some of Mueller's questions last week as part of the inquiry.