Russian opposition activists plan to publish significant elements of a report detailing military involvement in Ukraine that was being prepared by Boris Nemtsov before he was shot dead in Moscow.

Police investigating the opposition leader’s mafia-style killing on a bridge near the Kremlin searched his flat at the weekend and seized a computer believed to have contained the only copy of the report.

Ilya Yashin, another prominent member of the opposition, has toldThe Times that evidence collated by Mr Nemstov — including details obtained from parents of serving Russian soldiers who have died on Ukrainian soil — had been safeguarded, and a version of the report would be published.

Mr Yashin revealed the existence of the report after Mr Nemstov’s funeral, at the Troyekurovskoye cemetery, which was attended by his family, members of the opposition and hundreds of ordinary Russians.

“Nemtsov told me he was going to start work on a report entitledPutin and the War that would prove that Russian forces were present in Ukraine [and] took part in clashes with Ukrainian forces,” he said.

“Nemtsov told me that he was planning a trip to Ivanovo [northwest of Moscow] and that he planned to put the material together within a month in a report that would be distributed in a large number of copies,” he said.

“I have some ideas how to pick up the pieces of his report, but this is not something that can be done in a few days.”

He added that the issue had been complicated by the fact that investigators have seized Nemtsov’s documents, his computer and sealed his flat. “We have no way of getting to those documents, but have ideas about how to resurrect these documents and shall do that as soon as possible so that we can finish the work that was begun.”

Mr Yashin added that although it was too early to say how the opposition would respond to Mr Nemtsov’s death, he hoped that it would serve to unite different factions.

“Competition between different groups within the opposition only leads one way — to jail,” he said.

Reflecting on Mr Nemtsov’s death, Mr Yashin said : “He could have lived a comfortable and easy life. He chose a different path, He has left us as a hero.”

“We came because we feel ashamed of our country, of our people, that we let such a thing happen,” said Dmitry Afanasyev, another mourner . “Putin is to blame. But we are too.”

Vadim Prokhorov, Mr Nemtsov’s lawyer, also said that the politician was preparing to expose Russia’s role in the Ukraine conflict. He had turned to police after receiving anonymous death threats on social media, but the authorities had done nothing to investigate them. “Soon I’ll kill you,” read one such message.