More than a thousand people turned out in Cardiff to support a rally over the future of S4C.
The government wants the Welsh-language channel to be funded in future almost entirely through the BBC, and with a reduced budget, a move supporters see as a blow to its independence and its future.
Protestors at the rally in Cathays Park organised by Cymdeithas yr Iaith Gymraeg, the Welsh Language Society, heard calls for an independent review of the channel.
Deputy first minister for Wales, Ieuan Wyn Jones, was greeted with applause as he claimed S4C had been 'a pain' to successive UK governments. He said he had a message for David Cameron to transfer the powers and the money to run S4C to the Welsh people.
A spokesperson for the Department of Culture, Media and Sport said: "We are committed to a strong and independent Welsh language TV service, but have concluded that the S4C model is not sustainable in its present form. We believe a new partnership model with the BBC is the best way of securing the long-term future of the service."
Protest led to U-turn
The establishment of S4C in 1982 was a landmark in the long-running campaign to safeguard the Welsh language, spoken by over half a million people in Wales.
For years, protestors had been pressing for more Welsh TV programmes, and a dedicated channel. The new Conservative government of 1979 promised to establish one.
When, in 1980, the government made a U-turn, there were passionate protests, and veteran Plaid Cymru politician Gwynfor Evans threatened to fast to death. The government gave in, and S4C began broadcasting in 1982.
Since then, the channel, funded directly by the UK government, has had many successes, but in recent years, particularly following digital switchover, has been hit by falling viewing figures, and most recently by the proposed changes to its funding and governance, prompting today's protest
Earlier this month, the four main party leaders in Wales took the unusual step of writing jointly to the Prime Minister calling for an independent review of S4C.
Carwyn Jones, Ieuan Wyn Jones, Nick Bourne and Kirsty Williams said "political stewardship of S4C has been insufficiently vigilant."
In a joint statement First Minister Carwyn Jones and Deputy First Minister Ieuan Wyn Jones underlined the value of S4C.
"It is of incalculable importance to the further strengthening of the Welsh language as well as to the successes of Wales' creative industries sector," said the statement, "and it has an important role to play in ensuring much-needed plurality within Wales' media.
"We have laid out a constructive plan with the intention of ensuring a secure and independent future for S4C. We now await the Prime Minister's response."
BBC correspondents said it was "extremely significant" that the leaders of the Welsh Tories and Lib Dems had joined the leaders of Welsh Labour and Plaid in writing to Mr Cameron.
Downing Street confirmed that the letter arrived, and that the Prime Minister's office would respond in due course.
Among those speaking at Saturday's protest were Newport West MP Paul Flynn and Dave Donovan of the Bectu broadcasting union, who called for jobs to be safeguarded.
The rally, which Cymdeithas yr Iaith said had attracted up to two thousand people, also heard calls for supporters to refuse to pay the TV licence fee unless the independence of S4C is guaranteed.