Google is being threatened with legal action over the launch of its new social network, Google Buzz, amid furious claims that the service breaches users' privacy.

A week after launching the service with great fanfare and with high hopes that it could lure internet users' attention away from Facebook and Twitter, Google has found itself embroiled in a technical and public relations nightmare.

The pioneering internet company again apologised to users yesterday, and said it was working round the clock to roll out additional alterations to Buzz, on top of emergency changes imposed late last week and over the weekend.

Google said last Tuesday it was creating a vast social network from scratch by harnessing the 180 million users of its Gmail email service, converting parts of their contacts lists into Facebook-style "friends" who could share status updates, pictures and links.

But users revolted when they realised that their contacts could now see who they had been emailing – something that could reveal everything from private business relationships to romantic affairs.

Angry users have deluged Google with complaints and conversation in company chatrooms has turned to legal action. In Israel, one Gmail account holder has already reportedly launched a lawsuit that she hopes can be joined by other users. Amal Jaraisy said she had lodged the suit on behalf of people who "woke up one morning and found that the details of the people with whom they have open or covert contact are exposed to the entire world", according to the daily newspaper Haaretz.

Google had bypassed the careful testing that it usually insists upon for new products, trialling Buzz only using internal staff. Shelly Palmer, technology and media consultant and founder of Advanced Media Ventures, said the company had been caught unawares by the reaction of its public users.

"Anyone who understands the Google mindset could not have expected them to get this right," he said. "Everywhere they go, they try to apply mechanistic efficiency. They looked at Facebook and said, 'You have to invite people? How ridiculous! We'll just look at who you email most and hook up those people right now.' This wasn't a malicious attack on your privacy. It was just Google's attempt to create a social network with no fuss."

Google has issued repeated apologies to Gmail customers as the débâcle has unfolded and now makes it easier for users to keep their contacts private, to block certain people from following them and to unsubscribe from Buzz all together.

Todd Jackson, the Google product manager who a week ago had expressed his enthusiasm for the new service by declaring "Woohoo, Google Buzz launched!" was taking a different tone yesterday, saying the company was "Very, very sorry". In his latest blog post, he said: "We quickly realized that we didn't get everything quite right. We have been working hard ever since to improve things based on your feedback. We'll continue to do so."

Further changes under consideration include setting up Google Buzz as a standalone website, to further untangle it from Gmail. But, even as it was dealing with its public relations nightmare, Google was trying to look on the bright side. At least many of the Buzz users who were venting their fury were doing so on the new social network.