"It should be clear to everyone that Russia interfered in our 2016 presidential election process," Brennan said in testimony to the House Intelligence Committee, which is investigating possible collusion between Russia and President Donald Trump's campaign.
"And that they undertook the activities despite our strong protests and explicit warning they not do so," said Brennan, who served as CIA director from 2013 until January of this year when Trump took office.
Brennan told how he called the head of the Russian intelligence service, the FSB, on August 4 of last year.
"I said that all Americans, regardless of political affiliation or whom they might support in the election, cherished their ability to elect their own leaders without outside interference," Brennan said.
"I said American voters would be outraged by any attempt to interfere in the election," he added.
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Brennan's interlocutor denied any Russian interference but said he would pass on the warning to President Vladimir Putin, the ex-CIA chief said.
Brennan reiterated that the CIA detected in 2016 possible signs of collusion between Trump associates and Russian officials.
Those contacts are now being investigated by committees in both chambers of the US Congress and by recently appointed special counsel Robert Mueller, a former FBI director.
"I encountered and became aware of information and intelligence that revealed contacts and interactions between Russian officials and US persons involved in the Trump campaign," Brennan said.
He said he did not know if this amounted to outright collusion.
"I know there was a sufficient basis of information and intelligence that required further investigation" by the FBI, he added.
Trump vehemently denies any collusion and says he is the victim of an unprecedented witch hunt.
Brennan also addressed news reports that Trump, in an Oval Office meeting this month with the Russian foreign minister and ambassador, shared highly classified information provided by a US ally about an Islamic State group plot to bring down civilian airliners with bombs hidden in laptop computers.
Brennan said that if these reports are true, Trump violated two intelligence protocols.
First, he said, such intelligence is not shared with ambassadors but rather through intelligence channels.
And before such intelligence is shared, the country that provided it must be warned so as not to jeopardize sources or methods, Brennan said.
"It appears, at least from the press reports, that neither did it go in the proper channels nor did the originating agency have the opportunity to clear language for it," Brennan said.
"That is a problem."
