The former presidential candidate took aim at the US leader for failing to stand up to his Russian counterpart, whose country interfered with the 2016 election, according to American intelligence services.
The Vermont senator also criticised the president for “tearing” children and their families apart as part of his “zero tolerance” border policy.
“We say to Trump, instead of showing us your strength by tearing children from their families, where was your strength in standing up to Putin and Russia for undermining American democracy,” he told crowds at a large rally. "How courageous he is to tear children from their families, how cowardly he is to stand up to people who are trying to undermine not only American democracy but other aspects of our lives.
He added: “We have a president that’s so tough when he’s terrorising little children. When he was up against Vladimir Putin, he wasn’t so tough.”
Mr Trump faced criticism from both sides of the political divide following a joint press conference with Vladimir Putin earlier this week.
The American president appeared to side with Mr Putin's denial about Russian election meddling over his own intelligence community's assessment.
While discussing whether he thought Russia was behind hacks against the election, Mr Trump said: ”I don’t see any reason why it would be.“
He later backtracked on the remarks, saying: “I accept our intelligence community's conclusion that Russia's meddling in the 2016 election took place.”
But he added: “Could be other people also. A lot of people out there.”
The separation of children from their parents because of Mr Trump’s “zero tolerance” illegal immigration policy at the US-Mexico border has sparked a nationwide backlash.
The policy means all cases of adult illegal entry are sent for criminal prosecution, while their children are sent to different care facilities across the US.
Images emerged of youngsters in tent camps and holding facilities made up of cages.
Even Mr Trump’s wife Melania, said she “hates to see children separated from their families”.