Labour leader Jeremy Corbyn will join a march in Cardiff against plans to restrict public sector strikes.
Up to 400 people gathered at the steps of the National Museum for Saturday's protest against the UK government's Trade Union Bill.
Cardiff Central MP Jo Stevens said it was "illegal, illiberal and immoral" and "trying to silence working people".
Mr Corbyn will praise Welsh Labour ministers' efforts to stop the bill affecting devolved services in Wales.
In February, a leaked letter suggested UK ministers accepted the case for imposing the rules in Wales was "very weak".
As it stands, the law allows strikes to be called if the majority of those taking part in a ballot vote in favour.
The new bill, currently being considered by the House of Lords, would only allow strikes in key public services if they were backed by at least 40% of union members eligible to vote in the ballot.
The UK government has said it wants to ensure essential services are not disrupted at short notice by strikes supported by a small proportion of union members.
First Minister Carwyn Jones has threatened to defy Westminster by passing Welsh laws on strikes, even if it results in a Supreme Court battle over where responsibility lies.
Speaking before the event, Mr Corbyn said: "I congratulate the Welsh Labour government on their work to stand up for trade unionists in Wales.
"Welsh Labour, unlike the Conservatives in Westminster, know that strong public services rely on a well-trained, safe and decently-paid workforce."