Alexei Navalny, the Russian opposition leader, was recovering on Friday after being attacked with a chemical dye for the second time this year.
Mr Navalny was briefly hospitalised after an unknown assailant doused his face with a green dye known in Russia as zelyonka as he was leaving the office of his Foundation Against Corruption in Moscow on Thursday evening. Doctors said the politician suffered "chemical burns" to his right eye.
The attack on Mr Navalny comes as the Kremlin battles to curb a resurgence in opposition activism in the wake of nationwide anti-corruption demonstrations last month.
Mr Navalny, the driving force behind the protests, is planning to stand in the Russian presidential election next March even though a recent fraud conviction could be used by the authorities to bar him from the poll. Vladimir Putin has not yet declared his hand, but is widely expected to seek a fresh six-year term.
With more demonstrations looming over the May Day holiday weekend, Russia's prosecutor general banned three non-government organisations linked to Mikhail Khodorkovsky, the exiled former oil tycoon, and an arch Putin foe.
In a statement on Wednesday the prosecutor's office said the UK-registered Open Russia and its sister organisation, the Open Russia Civic Movement, aimed to foster protests and instability that "threatened the basic principles of the constitutional order of the Russian Federation and the security of the state".
The US-based Institute of Modern Russia, chaired by Pavel Khodorkovsky, Mr Khodorkovsky's son, was also blacklisted.
Open Russia has been urging Russians to join nationwide demonstrations against Mr Putin’s rule on Saturday.
In Moscow the authorities have banned the rally, but organisers have pledged to press ahead with plans to lead a march to Mr Putin’s office and demand that he does not run for re-election next year.
Russia’s opposition is fragmented among various groups and individuals that have squabbled among themselves over the years, undermining efforts to muster a united alternative to Mr Putin’s rule.
Mr Khodorkovsky, who has used Open Russia to try to build a viable opposition, has called on his supporters to back Mr Navalny’s presidential bid.
Mr Navalny has been campaigning across Russia in recent weeks in a drive to muster enough popular support to force the Kremlin to consent to his presidential candidacy. However, on Friday he cancelled plans to travel to Astrakhan on the Caspian Sea coast and stayed in Moscow to nurse his injured eye. "I woke at 4am to fly to Astrakhan and found my eye was hugely swollen and, instead of green, a nice dark red colour," he wrote on his Facebook page.
The attack would not alter his political strategy, he said, including plans to mobilise another nationwide anti-corruption protest on June 12th, the annual Russia Day holiday.
Zelyonka, often used as an antiseptic in Russia, has become a weapon of choice for anti-opposition activists.
Only this week an unknown assailant in the town of Saratov in southern Russia threw zelyonka, eggs and flour at Ilya Varmalov, an independent Russian journalist whose blogs are known for criticism of the authorities.
In a similar incident, Galina Sidorova, a Russian investigative journalist, was doused with zelyonka in the town of Yoshkar-Ola, 770km east of Moscow, where she was due to teach at a journalism course.
The New York-based Committee to Protect Journalists called on the Russian authorities to conduct a thorough investigation of the attacks. "The green tinge of dye may last for weeks, but the intimidating effect on Russia's independent journalists could last far longer," Nina Ognianova, the CPJ's Europe and Central Asia programme co-ordinator, said on Friday.