There have been violent clashes between police and demonstrators in Madrid, as millions of Spanish workers left their jobs in protest at the government and EU's ongoing economic policy of austerity.
Much of Portugal has also ground to a halt, in a co-ordinated general strike, which has resulted in many flights being cancelled.
Similar protests have been taking place across the EU today.
There have already been rallies and demonstrations in Greece, France and Italy.
However, the main focus today has been on Spain and Portugal, where strike action has been co-ordinated for the first time.
Six hundred flights were cancelled at Spanish airports. Aer Lingus cancelled flights to Madrid and Barcelona. Ryanair experienced delays.
While the major demonstrations are due tonight, protesters took to Portuguese and Spanish streets from early this morning.
There were clashes with police in Madrid with dozens of arrests.
In Belgium, European trade union officials visited several EU embassies, including Ireland's, ahead of Ireland taking-over the presidency of the EU next January.
Union leaders said they wanted the Irish presidency of the EU next January to deliver a job creation policy and ensure all countries had an input into European economic policy.
A letter was handed over to an embassy official.
Irish demonstration to go ahead on 24 November
The Irish Congress of Trade Unions says that while Irish trade unions support today's European day of action in protest at austerity measures, they will hold their own demonstration on 24 November.
Spokesman Macdara Doyle said the pre-Budget demonstration had already been organised when ICTU was notified of the day of action by the European Confederation of Trade Unions.
It was decided to go ahead with the 24 November march, as ICTU believed it would be difficult to call another mass demonstration in the space of ten days.
He said individual trade unions may support today's protest in Dublin's Dame Street.
The United Left Alliance is holding a solidarity protest with the general strike in southern Europe at 1pm at the Central Plaza, Dame Street.
The general strike will happen simultaneously in Portugal, Italy, Greece, Spain, Malta and Cyprus.