Nemtsov's RPR-Parnas and Navalny's Party of Progress said yesterday they are joining forces to put forward joint candidates at local elections this year and parliamentary polls next year.
In a statement the two parties accused the authorities under President Vladimir Putin of establishing an authoritarian system that had wiped out opposition, wrecked the economy and plunged the country into a conflict in Ukraine.
The fatal shooting of Nemtsov just yards from the Kremlin on February 27 had shown definitively that rule of law had broken down, the statement said.
They said that they expected other opposition parties to join their platform.
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Russia's beleaguered opposition has been sidelined during Putin's 15 years at the helm and does not have any representation in the Duma parliament.
The brazen murder of former deputy prime minister Nemtsov was the highest profile killing of an opposition figure during Putin's time in power.
Nemtsov's opposition allies accused the Kremlin of having had a hand in the killing but the authorities deny any involvement and have arrested five men on suspicion of carrying out a contract hit.
He was then hit with a string of corruption charges that he claimed were politically-motivated and held for months under house arrest.