Construction of the reservoir involved flooding the village of Capel Celyn and adjacent farmland, a deeply controversial move. Much of the anger was occasioned because the village was a strong-hold of Welsh culture and the Welsh language, whilst the reservoir was being built to supply Liverpool and parts of the Wirral with water, rather than Wales. The legislation enabling the development was also passed despite the opposition of 35 out of 36 Welsh Members of Parliament, with the 36th not voting. This led to an increase in support for the Welsh Nationalist party, Plaid Cymru, in the late 1950s and gave fresh impetus to Welsh devolution.
Although many doubted the wisdom of having an official opening, this took place on 21 October, 1965. Representatives came from Liverpool Council, and invitations were sent to all those with family links to the valley. In view of the anticipated protest, there was a strong police presence. The ceremony in fact lasted less than 3 minutes, for protesters had cut the microphone wires, and the chants of the hundreds of protesters made the speeches inaudible.
In October 2005, Liverpool City Council passed a public apology for the incident.
http://en.wikipedia.org/wiki/Llyn_Celyn