MOSCOW (AFP) - Tens of thousands of people petitioned President Vladimir Putin today to release a mother of seven accused of treason for having called the Ukrainian embassy about Russian troop movements.

Svetlana Davydova, who faces between 12 and 20 years in prison, is being held in the high-security Lefortovo jail in Moscow.

She was still breastfeeding her youngest child, a two-and-a-half-month-old girl, when she was arrested last month in the town of Vyazma, west of Moscow.

The case has shocked the country, and prompted over 50,000 Russians - including prominent authors, directors and TV celebrities - to sign an open letter to Mr Putin.

"Mr President, we ask you to be merciful towards a woman and mother of a large family," the letter reads. "We are hoping that investigation and a possible trial will be as open and just as possible and will be in accordance with the norms of the law."

Among the signatories are Ms Natalya Solzhenitsyna, widow of the Nobel literature prize-winner Alexander Solzhenitsyn, film director Andrei Zvyagintsev and actress Lia Akhedzhakova.

Mr Putin's spokesman Dmitry Peskov said the presidency had received the petition and would consider it.

Activists from the liberal party Yabloko picketed the headquarters of the FSB security service, successor to the Soviet-era KGB, whose investigator is in charge of the case.

Davydova, 36, who opposes the Ukraine conflict, phoned the Ukrainian embassy last April to allegedly report the military base located near her residential building in Vyazma had emptied, suggesting its soldiers might have been deployed across the border.

She also purportedly informed embassy staff she had overheard a serviceman saying troops of the Russian military intelligence service, the GRU, would be sent on a mission - presumedly to Ukraine.

The Ukrainian foreign ministry said it was not clear how Russian authorities had learned of the phone call, and expressed hope they were not eavesdropping on its Moscow embassy.

"We would not want to think this is the case," a foreign ministry spokesman said.

Observers call Davydova's case an embarrassment for authorities and their virtual blackout of any information regarding Russia's intervention in Ukraine.

Critics also describe Davydova's treatment as reminiscent of excesses by the repressive Soviet regime, and intended to signal the government's willingness to ramp up efforts to squelch dissent.

Ms Akhedzhakova, one of the country's most outspoken actresses, said she would do "absolutely everything" to help the woman.

"This is a signal of unthinkable monstrosity, this is 1938," she said on radio, referring to the height of Stalin-era repression.

Davydova's husband said he was grateful for the outpouring of support, adding he and other family members had been questioned by investigators earlier in the day.

"The entire family refused to testify against her," Mr Anatoly Gorlov said.

Authorities had previously said Davydova risked losing custody of her children. But on Tuesday, Russia's ombudsman for children's rights Pavel Astakhov said Davydova's husband was taking good care of the children, and that they would remain together as a family.

"Svetlana Davydova's younger children miss their mom very much," he said on Twitter, citing local authorities.

In 2012, Russia broadened the definition of high treason, alarming rights groups that said nearly anyone could fall afoul of the amended law.

Davydova is believed to be the first high-profile victim of the modified law.

"Practically anyone can now become its victim," said Mr Gorlov. "Now any housewive who looked out of her window can be accused of treason."