Tensions between Russia and Ukraine rose higher on Friday as casualties mounted from clashes between pro- and anti-Russian protesters in Donetsk and the Russian foreign ministry suggested it could intervene to protect lives.

The escalation came as the US secretary of state, John Kerry, met the Russian foreign minister, Sergei Lavrov, in London. Crimean residents will choose on Sunday whether to join Russia or to reinstate the constitution of 1992, under which Crimea enjoyed great autonomy from Kiev.

Donetsk, a largely Russian-speaking city in eastern Ukraine where many residents have close ties to Russia, declared a day of mourning on Friday after one person was killed and more than two dozen injured in a mass fight. The city has been the site of repeated standoffs between pro- and anti-Russian demonstrators.

On Friday, protesters from a pro-Russian demonstration fought with those from a rally "for a united Ukraine", resulting in the death of a 22-year-old man and injuries to a reported 26 people. Other reports said 28 people had been injured and that the young man had been stabbed to death.

Ukrainian media said pro-Russian protesters had attacked first, but the foreign ministry and Russian media reported that armed men had attacked peaceful pro-Russian demonstrators. In a statement released in response to the clashes, the foreign ministry said Kiev was not in control of the situation in the country and had failed to guarantee demonstrators' safety.

"Radical far-right gangs armed with traumatic firearms and clubs, who began to arrive in the city yesterday from other regions of the country, attacked peaceful protesters who came out on the streets to express their attitude towards the destructive position of the people who call themselves the Ukrainian government," the foreign ministry said in a statement.

The statement also hinted that Russian forces could intervene in eastern Ukraine to protect Russians there, the same justification used for sending troops to occupy key facilities in Crimea.

"Russia recognises its responsibility for the lives of countrymen and fellow citizens in Ukraine and reserves the right to take people under its protection," it said.

The foreign ministry website was not working on Friday afternoon, but much of the statement was carried by Russian news agencies.

The head of Ukraine's security service wrote on his Facebook page on Friday that four people had been detained in connection with the violence in Donetsk and that "these detainments are only the beginning".

Russian troops and armoured vehicles had massed on the border with eastern Ukraine on Thursday, alarming Kiev, where the acting president, Oleksandr Turchynov, said in a statement that the Russian forces were "ready to intervene in Ukraine at any time".

The Russian defence ministry admitted in several statements that at least 10,000 troops had gathered in provinces along the border, but said they were there only to participate in intensive exercises. Moscow also ordered six Sukhoi-27 fighter jets and three transport planes into Belarus, located on Ukraine's northern border, to head off what the Belarusian president, Alexander Lukashenko, said was a potential Nato threat.

In response to the buildup, the German leader, Angela Merkel, warned Moscow in her strongest language yet that it risked massive political and economic damage if it refused to change course on Ukraine.

Donetsk resident Anton Nagolyuk, who was present during the clashes, said the attacks originated from the pro-Russian part of the demonstrations. The pro-Russian protesters had arrived at Lenin Square before the pro-Ukrainian ones, and police at first kept the two sides apart. The pro-Russian side threw eggs and firecrackers, and when the rally ended they started to beat pro-Ukrainian demonstrators, Nagolyuk said. The man who was killed was from the pro-Ukrainian side, he said.

"Some of the people definitely came from Russia to provoke people, but I don't know exactly how many of them there were," Nagolyuk said.

"It's true there are many Donetsk residents among the [pro-Russian demonstrators], but it seems to me the most active ones are Russian," he added.

According to Nagolyuk, many of his fellow residents fear Russia will invade their part of the country. "They were deliberately waiting for a death, an excuse to bring their troops and tanks into Donetsk, Lugansk and Kharkov," he said. "It seems to me almost everyone is afraid of war and Russian troops."