A clear majority of Americans are concerned about the actions and operations of the National Security Agency (NSA) and want the intelligence body to be subjected to further review and greater congressional oversight, a Guardian poll has found.

In the opinion poll, conducted for the Guardian by Public Policy Polling, two-thirds of voters who responded said that in the light of a week-long series of leaked disclosures about the NSA's surveillance activities they wanted to see its role reviewed. Only 20% thought there were no grounds for further review, while 14% could not say either way.

In a separate question, 56% said that they believed Congress had failed to conduct sufficient oversight of the NSA, which is a branch of the department of defence charged with collecting and analysing national security information. Recent disclosures by the Guardian have unveiled the NSA's vast data-mining programs of telephone records and other digital communications involving millions of Americans.

The revelations originated with a former contract worker for the NSA, Edward Snowden, who said he decided to blow the whistle on the agency's digital dragnet operations because he wanted the public to know about the "federation of secret law, unequal pardon and irresistible executive power that rule the world". The poll suggests that his stated ambition has, at least for now, been achieved: some 90% of those surveyed said they had heard about the recent news involving the NSA's collecting and storing of Verizon phone records and gaining access to data from major internet companies, and 61% said they believed a fresh debate was needed over the balance between privacy and security.

The poll shows that a substantial majority of Americans – 60% – want their government to be more open about its data collection so that the public can understand what is going on. A much smaller proportion, only 35%, said they agreed that the government needs to keep the data it collects secret in order to protect national security.

Photograph: guardian.co.uk

Most American voters – 58% – are also fearful about private firms acting as contractors to the defence department have too much access to state secrets. Only 8% of respondents said they thought government contractors had too little access.

Snowden worked most recently for the private firm Booz Allen Hamilton, doing contract work through that company for the NSA in Hawaii.

Despite these areas of relative agreement, Americans are much less firm in their opinions when asked about official data gathering activities. Some 44% of respondents approve of the government collecting internet and/or phone data from them, and a relatively close 50% think it is a bad idea.

Similar results were found when the sample was asked what they thought of the government's collection of metadata, with 40% approving and a 50% disapproving. The term was explained in the survey as being the collection of characteristics such as length of a phone call and who called whom but not the content of the phone call itself.

The Obama administration has attempted to justify the NSA's actions in the light of the recent leaks by saying the agency only skims metadata information rather than listening into conversations. Advocates of greater openness in official affairs say that this is misleading.

"It's not surprising that people are more ambivalent about metadata because many people have little knowledge about how deeply personal metadata can be. It can be a window into every aspect of your life – who you are speaking to, for how long, where from – it's a database of everything you do," said Kurt Opsahl, senior staff attorney at the campaign for online rights, the Electronic Frontier Foundation.

One group more than any other has intense qualms about the expansion of executive powers in data sweeping: young Americans. Among 18- to 29-year-olds, a commanding 81% want to see the NSA's role reviewed and 69% think there needs to be a wider debate about privacy versus security.

The poll also indicates that the disclosures could have political ramifications for Obama. Nearly half – 48% – of American voters say they are less likely to support Obama because of the recent revelations on data-gathering, while only 17% say they are more likely to support him.

This split is less pronounced among Obama's Democratic base, with 25% of Democrats saying they were more likely to support Obama against 28% who said the opposite. Obama's Democratic base, especially among black voters, generally expressed less concern about the NSA disclosures than Republican-leaning voters.

For instance, 68% of those identifying themselves as Republican said they thought Congress had not done enough oversight of the agency, while only 40% of Democrats were similarly worried.

There was one area, however, of bipartisan agreement. Both Democratic- and Republican-leaning respondents replied by 59% that they felt the government needed to be more open about its data collection activities.

• Public Policy Polling surveyed 512 registered voters between 10-11 June 2013 on behalf of the Guardian. The firm did not use cell phones to reach the sample.