Iran's most prominent human rights activist was released from jail on Wednesday along with several other political prisoners in what appears to be the most tangible sign of change yet under the new Iranian president, Hassan Rouhani.

Ahead of Rouhani's eagerly awaited visit to the UN general assembly in New York next week, Nasrin Sotoudeh, who has been likened to Aung San Suu Kyi, was driven from Evin prison in Tehran to her house in another part of the Iranian capital and told she did not need to return to jail.

"They were quite certain this time that I'm freed and I don't need to go back," the 50-year-old women's rights activist told the Guardian by phone from her home.

Opposition website Kaleme reported on Wednesday that seven other women political prisoners had also been released in the previous 24 hours, including the dissident journalist Mahsa Amrabadi, and at least four men, including reformist politicians Feizollah Arabsorkhi, Mirtaher Mousavi and former deputy foreign minister Mohsen Aminzadeh.

"In the past, when I was granted prison leave they used to give me a document, this time they gave me nothing," said Sotoudeh, who last October was awarded the European parliament's most prestigious human rights award, the Sakharov prize for freedom of thought, which has previously been won by Suu Kyi and Nelson Mandela.

"My goals and mentality are the same as before, I haven't changed," Sotoudeh insisted, adding that like other lawyers she would still work "to restore justice and defend the rights of protesters".

The prisoner releases have come amid increasing signs of a political opening-up in the Islamic republic following Rouhani's inauguration last month and as he prepares for his UN visit, which many have suggested may be the scene for a historic meeting between the Iranian president and Barack Obama.

Iran's supreme leader on Tuesday struck a conciliatory tone by saying there was room for flexibility in diplomacy, which many have interpreted as his public backing for Rouhani's new attempt to improve Iran's relations with the west, especially conducting direct talks with the US. Iranians, reacting on social networks, linked the releases to Khamenei's talk of "leniency" on Tuesday. In the light of recent developments in Iran, its national currency, the rial, has steadily recovered value against the dollar.

The UK foreign secretary, William Hague, welcomed the news.

He said: "I welcome reports that a number of Iranian political prisoners, including human rights lawyer Nasrin Sotoudeh, have been released from prison today. This will come as a great relief to their families and supporters around the world. The UK hopes to see further improvements in Iran's human rights record under President Rouhani's new government."

Suzanne Maloney, an Iran expert at the Brookings Institution in Washington, said there has been a fear Rouhani would be more impressive with his rhetoric than actions but the release of prisoners provided another piece of evidence that he would be different.

"At least we can see that the president is following through on his campaign promises, and that this election is having tangible, positive impact on the lives of individual Iranians. As Iran's delegation arrives in New York … Rouhani's international credibility will be strengthened by these developments."

Ali Vaez of the International Crisis Group echoed Maloney. "The Iranian government is paving the ground for Rouhani's arrival on the world stage. Undoubtedly, combining concrete actions with words enhances the new president's credibility and proves that he can deliver on his promises."

However Trita Parsi, the president of the National Iranian American Council, warned: "If the west fails to appreciate Rouhani's flexibility now, it will have to deal with the inflexibility of the conservatives in a few months."

The country's opposition leaders, Mir Hossein Mousavi and Mehdi Karroubi, have been under house arrest for more than two years. But Karroubi's son, Mohammad-Taghi, said last week the house arrests of the two men have been eased recently and they have been allowed more family visits.

Analysts believe the prisoner releases will pave the way for Mousavi and Karroubi to be granted their freedom.

Sotoudeh fell foul of the authorities after representing several political activists and highlighting the execution of juveniles in Iran. After her arrest in September 2010, she was initially sentenced to 11 years in jail for "propaganda against the regime" and "acting against the national security", but an appeals court reduced the sentence to six years.

Prominent international human rights organisations have said that charges against Sotoudeh were fabricated and politically motivated.

While in jail, Sotoudeh, the mother of two, embarked twice on long hunger strikes in protest at the mistreatment of her 12-year-old daughter and her husband, Reza Khandan, at the hands of the authorities.

Sotoudeh lost both her parents while in prison. She was not allowed to attend the funeral of her father, who died two weeks after she was arrested, but she was present at her mother's burial ceremony for a few hours in December 2012.

Faraz Sanei of Human Rights Watch welcomed Sotoudeh's release, but added: "The burden is on Tehran to prove that this is more than just a symbolic gesture.

"If the Iranian government is truly serious about real reform it needs to take immediate and concrete steps leading to the unconditional release of hundreds of other political prisoners currently languishing in Iran's prisons. It also needs to ensure that those who are freed will not continue to suffer targeting at the hands of security forces and judicial authorities."