He told 150 sector leaders at Third Sector's annual Britain's Most Admired Charity awards that he was trying to create a culture in Westminster that was conducive to campaigning.
"Whatever the views of ministers about your specific campaigns, it is massively in the interests of progressive politicians to champion your campaigning role - not by picking causes, but by supporting your right and ability to campaign and by opening up government to your voice," he said.
"Politicians, whether they are local or national, should understand, celebrate and respect your right to campaign - and understand that there is no contradiction if you are helping deliver services with a local authority and you are also campaigning against that local authority.
"It's not comfortable for government to have the third sector ranged against it, but that is part of what democracy is about."
Miliband said many charities were still uncertain about how much campaigning they could do, despite the rewriting of the Charity Commission's guidelines, CC9. He welcomed the commission's plan to issue further guidance.
"I am conscious that many local charities feel it is very hard to criticise a local authority because of fear their funding may suffer," he said.
"I have discussed this with John Stoker, the Commissioner for the Compact. I know he is interested, and I hope this will be an early focus for his work."
The minister said government should not expect the sector to speak with one voice, but added that its collective power was enormous and sometimes underused.
"We have seen its success in countless areas, and I believe it could have even greater power," he said.
Miliband was questioned about the effects of the Communications Act, which forbids political advertising and is said to hamper charity campaigns, and of the Serious Organised Crime and Police Act, which outlaws demonstrations near Parliament without a week's notice. He agreed TV had special difficulties and pointed out that demonstrations were still taking place.
He was also asked about diverting to charities some of the money used for public education campaigns; he replied that this already took place but could happen more.
- See Politics, page 7.