Prime Minister Malcolm Turnbull has condemned the "thuggish harassment" of former prime minister John Howard by CFMEU members rallying in Sydney's CBD.
Mr Howard was OK despite needing to be "rescued by police" on Thursday after inadvertently coming across a group of angry union members protesting against cuts to penalty rates.
The 77-year-old former Liberal leader had been attending a conference on Pitt Street when he walked out onto the street to find dozens of people shouting, booing him, holding up CFMEU flags and making profane gestures.
Footage on social media shows Mr Howard having to walk several hundred metres along the city street, flanked by a number of NSW police, as noisy protesters repeatedly chanted "The workers, united, will never be defeated".
Mr Howard appeared to take the incident in his stride, continuing to walk along the streets until the mob returned to the rally.
While the former PM had initially been shocked, NSW Police "came to the rescue" to move him away and he was "fine", a spokesman for Mr Howard told AAP.
Mr Turnbull later tweeted: "I condemn CFMEU's thuggish harassment of John Howard today. A reminder of why ABCC is needed. Bill Shorten should condemn CFMEU as well."
The CFMEU, supported by other unions, held rallies in capital cities on Thursday to protest against what the union says are the federal government's "attacks on working people".
ACTU president Ged Kearney, who didn't see the incident but heard about it, told AAP "there is a lot of anger against John Howard because of WorkChoices".
"It's the sort of feeling - you've got Work Choices Mark II with this (Turnbull) government," she told AAP on Thursday.
"The good thing about today was that thousands and thousands of workers turned out to stand together."
The CMEU issued a statement arguing Mr Howard "walked into the Sydney rally".
"The rally was peaceful but loud," the union said.
"From the footage we have seen, it appears some protesters made their views known to Mr Howard about anti-worker laws he introduced while prime minister, as a result of which he lost an election and his seat."