Greenpeace activists have scaled an emblematic Madrid office building to hang a banner protesting a free trade deal being negotiated between the U.S. and European Union.

Greenpeace said the Trans-Atlantic Trade and Investment Partnership, or TTIP, threatens to undo many health, social and environmental protection measures.

Many activist groups oppose the talks and criticize the secretive manner in which they have been conducted. Greenpeace said they should be held under the umbrella of the United Nations or as part of climate change talks.

Proponents argue that TTIP tariff reductions and regulation changes would give a much-needed boost to businesses.

The activists Tuesday hung the banner reading, "No to the TTIP" from one of the two Gate to Europe towers that lean toward each other on Madrid's Plaza de Castilla.