Jeremy Corbyn has accused "unscrupulous bosses" of using technology to undermine workers' rights in a speech to trade union members.
The Labour leader told the TUC conference that the rise of the so-called gig economy has harmed workers' mental health.
He urged young people to join a trade union to protect their rights.
At the same event, the leader of Unite told the BBC co-ordinated strikes on public sector pay were "highly likely".
Mr Corbyn used his address to the trade union conference in Brighton to warn of the "epidemic of low pay" which he said was blighting the British economy and the lives of millions of workers.
He also hit out at companies that use temporary and short-term contracts, saying they were a "source of continuous worry and insecurity for millions of people".
The Labour leader also said the gig economy, which is presented as "modern and dynamic", denies "both employees and customers basic protections".
Young people should ignore the way trade unions are "demonised" and safeguard their rights by joining one, Mr Corbyn said.
Labour winning a general election would be "simply not enough" without increased participation in trade union activity, he said.
"Politics is about power and democratic politics is about putting power in the hands of the many not just the few.
"That principle applies in the workplace too," he added.
The issue of public sector pay has dominated the conference, with the TUC warning the government against "cherry-picking" some workers for pay rises.
The government announced on Tuesday that the cap would end next year but unions responded angrily to the offer of a 1.7% deal to prison officers this year and a 1% rise in basic pay and a 1% bonus for police officers.
Unite general secretary Len McCluskey told the BBC he was willing to break the law to go on strike over the issue in the coming months.
Since March, successful strike ballots have had to achieve a 50% turnout for legal industrial action to go ahead.
Mr McCluskey said he would disregard what he called an "artificial threshold" if his members backed a strike over pay in insufficient numbers in the near future.
In response, a Downing Street spokeswoman said it was "irresponsible for unions to condone breaking the law".
Also in his TUC conference speech, Mr Corbyn set out his party's "jobs first" plan for Brexit, the day after he was accused of presenting a muddled policy on single market membership.
Free movement rules will end and be replaced with "fair rules and reasonable management of migration", he said.
"We must never let ourselves be duped and divided. It isn't migrants who drive down wages and conditions but unscrupulous employers, supported by a government that slashes rights and protections at work whenever it gets the chance."
But the Conservatives said Mr Corbyn still appeared confused over whether the UK could remain a member of the single market after Brexit and that this demonstrated the Labour leader "cannot be trusted to make decisions on behalf of the UK and would bring the UK to a standstill".