New claims emerged last night over the extent that US intelligence agencies have been monitoring the mobile phone of Angela Merkel. The allegations were made after German secret service officials were already preparing to travel to Washington to seek explanations into the alleged surveillance of its chancellor.

A report in Der Spiegel said Merkel's mobile number had been listed by the NSA's Special Collection Service (SCS) since 2002 and may have been monitored for more than 10 years. It was still on the list – marked as "GE Chancellor Merkel" – weeks before President Barack Obama visited Berlin in June.

In an SCS document cited by the magazine, the agency said it had a "not legally registered spying branch" in the US embassy in Berlin, the exposure of which would lead to "grave damage for the relations of the United States to another government".

From there, NSA and CIA staff were tapping communication in Berlin's government district with high-tech surveillance. Quoting a secret document from 2010, Der Spiegel said such branches existed in about 80 locations around the world, including Paris, Madrid, Rome, Prague, Geneva and Frankfurt. Merkel's spokesman and the White House declined to comment on the report.

The nature of the monitoring of Merkel's mobile phone is not clear from the files, Der Spiegel said. It might be that the chancellor's conversations were recorded, or that her contacts were simply assessed.

Ahead of the latest claims , the German government's deputy spokesman, Georg Streiter, said a high-level delegation was heading to the White House and National Security Agency to "push forward" investigations into earlier surveillance allegations.

Meanwhile several thousand people marched to the US Capitol in Washington yesterday to protest against the NSA's spying programme and to demand a limit to the surveillance. Some of them held banners in support of Edward Snowden, the former CIA contractor who revealed the extent of the NSA's activities.

The march attracted protesters from both ends of the political spectrum as liberal privacy advocates walked alongside members of the conservative Tea Party movement.

The delegation will include senior officials from the German secret service, according to German media reports.

Germany and Brazil are spearheading efforts at the UN to protect the privacy of electronic communications. Diplomats from the two countries, which have both been targeted by the NSA, are leading efforts by a coalition of nations to draft a UN general assembly resolution calling for the right to privacy on the internet. Although non-binding, the resolution would be one of the strongest condemnations of US snooping to date.

"This resolution will probably have enormous support in the GA [general assembly] since no one likes the NSA spying on them," a western diplomat told Reuters on condition of anonymity. The Brazilian president, Dilma Rousseff, had previously cancelled a state visit to Washington over the revelation that the NSA was scooping up large amounts of Brazilian communications data, including from the state-run oil company Petrobras. The drafting of the UN resolution was confirmed by the country's foreign ministry.

The Associated Press quoted a diplomat who said the language of the resolution would not be "offensive" to any nation, particularly the US. He added that it would expand the right to privacy guaranteed by the international covenant on civil and political rights, which went into force in 1976.

The draft would be sent next week to the general assembly subcommittee on social, humanitarian, cultural and human rights issues, and be put to the full general assembly in late November.

Germany and France demanded on Thursday that the Americans agree to new transatlantic rules on intelligence and security service behaviour by the end of the year. Merkel added that she wanted action from Obama, not just apologetic words.

British and US civil liberties groups on Saturday added their voices to the criticism of snooping by both UK and US intelligence services after the Guardian revealed that the British intelligence agency GCHQ repeatedly said it feared a "damaging public debate" on the scale of its own activities.

Shami Chakrabarti, the director of Liberty, and Anthony Romero, executive director of the American Civil Liberties Union, issued a joint statement, saying: "The Guardian's publication of information from Edward Snowden has uncovered a breach of trust by the US and UK governments on the grandest scale. The newspaper's principled and selective revelations demonstrate our rulers' contempt for personal rights, freedoms and the rule of law.

"Across the globe, these disclosures continue to raise fundamental questions about the lack of effective legal protection against the interception of all our communications. Yet in Britain that conversation is in danger of being lost beneath self-serving spin and scaremongering, with journalists who dare to question the secret state accused of aiding the enemy.

"A balance must of course be struck between security and transparency, but that cannot be achieved while the intelligence services and their political masters seek to avoid any scrutiny of, or debate about, their actions."

"The Guardian's decision to expose the extent to which our privacy is being violated should be applauded and not condemned."

Nick Pickles, director of Big Brother Watch, said the fact GCHQ had doubts about the legality of its surveillance "reinforces the public interest in the disclosures about what has taken place in America and closer to home

"Parliament never legislated to allow the scale of interception that has been exposed, with laws written long before widespread broadband internet access or Facebook existed. There is a clear and overwhelming need for a fundamental review of our legal framework."

"If companies are handing over customer data or access to their equipment when there is no legal authority, then those businesses may well have broken the law. This should be urgently investigated by the information commissioner."

Defending the NSA's actions, the US administration has insisted that it is necessary to intercept vast amounts of electronic data to effectively fight terrorism, but the White House has said it is examining countries' concerns as part of an ongoing review of how the US gathers intelligence.