They saw it as continuing interest in the sector by political parties and an opportunity to hold the Government to pledges on key questions such as better funding and consultation.
But some argued that the Prime Minister's speech took the wrong tack, and warned of potential disadvantages in too close a relationship between the sector and government.
The speech, widely seen as a pre-election appeal to the middle ground, called for a new kind of consensus politics and policy making.
Stuart Etherington, chief executive of the NCVO, said: "Politicians look at the decline in political membership and voting and contrast that with the desire that people have to get involved in ground-breaking campaigns.
"So they ask if engagement in civil society is a route to creating a rounded culture where people do participate more in the political process."
But Etherington added that there could be dangers for the sector in entering a "big tent" with government. "You can be drawn into a process that can deflect you from your purposes, and you can end up doing deals that can cause problems," he said.
Stephen Bubb, head of chief executives body Acevo, said Brown's choice of venue showed that he believed in the power of the sector, and was a big boost for voluntary organisations.
"I can't remember another time in the past decade when a keynote speech like this was made to a voluntary sector audience," he said.
Debra Allcock Tyler, chief executive of the Directory of Social Change, warned the Prime Minister against new initiatives.
"Increased government involvement stifles innovation and flexibility," she said. "Brown needs to put his money where his mouth is when he talks about empowering the sector; he must provide the flexible resources for the sector to do what it does best."
John Low, chief executive of the Charities Aid Foundation, said the Prime Minister's plans for new citizens' forums and summits might be misguided.
"He's trying to create new mechanisms for consulting people, and not tapping into what the voluntary sector actually does," said Low. "It might be a trick missed."