Russia's opposition leaders have expressed both devastation and and fury after charismatic former deputy prime minister Boris Nemtsov was killed in a drive-by shooting on a Moscow bridge.

His death comes just a day before a planned protest against President Vladimir Putin's rule.

The death of Mr Nemtsov, 55, ignited a fury among opposition figures who attacked the Kremlin for creating an atmosphere of intolerance of any dissent.

Mr Putin quickly offered his condolences and called the murder a provocation, ordering Russia's law enforcement chiefs to oversee the probe.

"Putin noted that this cruel murder has all the makings of a contract hit and is extremely provocative," presidential spokesman Dmitry Peskov said.

Mr Nemtsov was a key critic of Mr Putin, assailing the government's inefficiency, rampant corruption and the Kremlin's policy on Ukraine, which has strained Russia-West ties to a degree unseen since Cold War times.

In Britain, a Foreign Office spokeswoman said: "We are shocked and saddened by news that former Russian deputy prime minister Boris Nemtsov has been shot and killed in Moscow.

"Our thoughts are with his family and we offer our condolences to them. We deplore this criminal act. Those responsible must be brought to justice. We will continue to follow the situation closely."

Ukrainian president Petro Poroshenko said on Facebook that he was shocked by the killing of Mr Nemtsov, who he called a friend and a "bridge" between the two countries. He said he hoped the killers would be punished.

Mr Nemtsov's lawyer Vadim Prokhorov said the politician had received threats on social networks and told police about them, but authorities did not take any steps to protect him.

The Interior Ministry, which oversees Russia's police force, said Mr Nemtsov was shot four times from a passing car as he was walking on a bridge just outside the Kremlin shortly after midnight.

Ministry spokeswoman Yelena Alexeyeva said Mr Nemtsov was walking with a woman acquaintance, a Ukrainian citizen, when a vehicle drove up and unidentified attackers opened fire. The woman was unhurt.

US president Barack Obama called on Russia's government to perform a "prompt, impartial and transparent" investigation to bring the perpetrators to justice. He called Mr Nemtsov a "tireless advocate" for the rights of Russian citizens.

As Mr Nemtsov's body was removed amid chimes from Kremlin bells nearby, Mikhail Kasyanov, a former Russian prime minister now also in opposition, told reporters: "In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin."

"The country is roiling into the abyss."

Mr Nemtsov served as deputy prime minister in the 1990s and once was seen as a possible successor to Boris Yeltsin, Russia's first elected president.

After Mr Putin was first elected in 2000, Mr Nemtsov became one of the most vocal critics of his rule, helping to organise street protests and writing extensively about official corruption.

He was one of the organisers of the Spring March opposition protest set for Sunday, which comes amid a severe economic downturn in Russia caused by low oil prices and Western sanctions.

Opposition activist Ilya Yashin told Ekho Moskvy radio that he last spoke with Mr Nemtsov two days before the killing. Mr Nemtsov was working on a report presenting evidence that he believed proved Russia's direct involvement in the separatist rebellion that erupted in eastern Ukraine last year.

Ukraine and the West have accused Russia of backing the rebels there with troops and weapons. Moscow has denied the accusations, but large numbers of sophisticated heavy weapons in the rebels' possession has strained the credibility of its denials.

Mr Yashin said he had no doubt that Mr Nemtsov's murder was politically motivated.

"Boris Nemtsov was a stark opposition leader who criticised the most important state officials in our country, including President Vladimir Putin. As we have seen, such criticism in Russia is dangerous for one's life," he said.

Political analyst Stanislav Belkovsky told the radio station that he did not believe that Mr Nemtsov's death would in any way serve Mr Putin's interests.

"But the atmosphere of hatred towards alternative thinkers that has formed over the past year, since the annexation of Crimea, may have played its role," he said, referring to the surge of intense and officially-endorsed nationalist discourse increasingly prevalent in Russia since it annexed Ukraine's Crimean Peninsula.

Irina Khakamada, a prominent opposition figure who co-founded a liberal party with Mr Nemtsov, blamed a climate of intimidation and warned that the murder could herald a dangerous destabilisation.

"It's a provocation that is clearly not in Putin's interests, it's aimed at rocking the situation," she said.

In an interview with the Sobesednik newspaper earlier this month, Mr Nemtsov said his 86-year-old mother was afraid that Mr Putin could have him killed for his opposition activities. Asked if he had such fears himself, he said: "If I were afraid I wouldn't have led an opposition party."

Speaking on Ekho Moskvy radio just a few hours before his death, he harshly criticised Mr Putin for plunging Russia into crisis by his "mad, aggressive and deadly policy of war against Ukraine".

"The country needs a political reform," he said. "When power is concentrated in the hands of one person and this person rules for ever, this will lead to an absolute catastrophe, absolute."

Garry Kasparov, a former world chess champion who worked with Mr Nemtsov to organise protests against Mr Putin and now lives in the United States, said the killing showed Mr Putin and those who supported him were lying when they said their popular support was strong.

"If you have 86% support, why do you kill someone like Boris?" he said. "He maybe can reach two million people online at best. A demonstration brings out a hundred thousand people at most. So if you are so confident, why do that?"