Nemtsov's family, including his mother and children, are standing in front of the casket. A wreath from the Russian president has been delivered to Sakharov Centre.
Russian Prime Minister Dmitry Medvedev's spokesperson said the government is represented by Deputy Prime Minister Sergey Prikhodko.
US Ambassador to Russia John Tefft is also attending the visitation.
After attending the visitation services, the head of Rosatom state nuclear energy corporation, Sergei Kiriyenko, said he hoped "the law enforcement agencies will manage to quickly establish, at minimum, the perpetrators and contractors of that political murder".
Russian opposition activist Alexey Navalny had asked the court to suspend his house arrest for one day to attend the ceremony but was denied.
Nemtsov, co-chair of RPR-Parnas and deputy of the Yaroslavl regional duma, was killed as he was crossing Moskvoretsky Bridge in view of the Kremlin overnight to Saturday.
The funeral for the opposition politician will take place later on Tuesday at Moscow's Troyekurovskoye cemetery.
