A campaign against a major trade deal is being stepped up this week amid fresh warnings that jobs and public services were being threatened.
Opponents claimed that a proposed deal between the European Union and the United States, known as the Transatlantic Trade and Investment Partnership (TTIP), is the result of "secret" negotiations between US officials and the European Commission.
Protests will be held in Birmingham, Manchester and London this week, followed by a number of demonstrations across Britain on Saturday.
War on Want said the treaty would see US firms bidding for public sector contracts in this country, "entrenching" privatisation.
Jeff Powell, campaigns and policy director at War on Want, said: "It is a scandal that discussion of such a dangerous threat to jobs, public services and democratic safeguards is being held in secret.
"This campaign will show that people from all walks of life in Britain will fight every inch of the way to halt a deal that puts corporate greed before public need."
Unite general secretary Len McCluskey, said:"David Cameron has shown that he's willing to go to Europe to defend bankers' bonuses, now he must use his powers to defend our cherished NHS.
"US health companies will even have the right to sue a future UK government in secret courts if politicians try to reverse privatisation.
"The most significant effect will be felt in health, enabling US healthcare multinationals and Wall Street investors to sue the UK government in secret courts if it attempts to reverse privatisation.
"David Cameron can exempt the NHS from these trade negotiations. Unless the Prime Minister acts, bureaucrats in Brussels and Washington will make the sell-off of our NHS irreversible."
Unite said the trade agreement was aimed at increasing the power of multinational investors such as big businesses and hedge funds, while reducing regulation of these organisations.
The union said the bilateral trade agreement would create a single market between the European Union and the United States, removing barriers between public assets and multinational corporations.
TTIP could make it impossible for any European government to reverse the privatisation of public assets, such as the railways or healthcare, said campaign groups.
Business Secretary Vince Cable said: "There is no question of TTIP being used to promote an ideological agenda for the NHS. There is no suggestion whatever that the TTIP negotiations could be used to undermine the fundamental principles of the NHS or advancing privatisation.
"The NHS will always be there for everyone who needs it, funded from general taxation, free at the point of use.
"Our focus for health is to enable our world-class pharmaceutical and medical devices sectors to benefit from improved access to the US market, increasing growth and employment in the UK."