The group, Transparency International, said in a voluminous report released in Brussels that many European countries had gaps in their laws governing lobbying, the financing of political campaigns and public access to information.
"Across Europe, many of the institutions that define a democracy and enable a country to stop corruption are weaker than often assumed," said Cobus de Swardt, the group's managing director. "This report raises troubling questions at a time when transparent leadership is needed as Europe tries to resolve its economic crisis."
The report said four countries in southern Europe — Greece, Italy, Portugal and Spain — "are shown to have serious deficits in public sector accountability and deep-rooted problems of inefficiency, malpractice and corruption."
It said that, among the European Union's new members, Bulgaria and Romania continued to cause the most concern. While both countries have enacted numerous anti-corruption laws, as required by the EU, "this flurry of legislative activity has not been accompanied by the widespread adoption of ethical norms, actions and behaviour," the report said.
By contrast, Transparency International said Denmark, Norway and Sweden were best protected against corruption, with strong watchdogs, auditors, justice systems and law enforcement agencies.
But it said that even some of the countries presumed to be "integrity leaders" have left themselves open to possible corruption.
Sweden and Switzerland have no mandatory regulation of the financing of political parties, the report said. It added that the party financing systems in Denmark, Germany and the U.K. were "far from exemplary."
The group also called on the European Union to set an example by adopting strict rules for its own institutions.
Transparency International said it assessed more than 300 national institutions in 25 European countries, and concluded that many governments were not sufficiently accountable for public contracts, which it said were worth €1.8 trillion ($2.24 trillion) a year.
It said only two of the countries assessed — Norway and the U.K. — adequately protected whistleblowers who spoke out against wrongdoing.
And there were other "corruption risks," the group said. Twelve countries in all had no limits on political donations by individuals and 17 lacked codes of conduct for members of parliament, the report said.
In 20 of the 25 countries, the group found barriers to people who want access to public information.
Don Melvin can be reached at http://twitter.com/Don_MelvinSuggest a correction