Traffic is being restricted across the capital for the demonstration by the Right 2 Water campaign - which will continue until 4 o'clock this afternoon.
TD's taking part say the protest is a message to the Government that the people have had enough austerity - and a General Election should be called.
Crowds have been gathering outside Leinster House here for several hours - with protesters making their way from across the country many from Donegal. As many as 20 busloads left the county this morning.
Gardaí say tens of thousands are taking part - while the Right 2 Water campaign have put the figure at 100-thousand.
The centrepoint of the protest is a stage set-up at Merrion Square - where figures including Sinead O Connor and the Detroit Water Brigade from the US will speak to the crowd.