Nikita Tikhonov and his partner Yevgenia Khasis went on trial last week over the killings of lawyer Stanislav Markelov and journalist Anastasia Baburova, who were shot dead in January 2009.
Both have pleaded not guilty.
Witness Larisa Yermakova testified today that she saw Tikhonov several seconds after she heard the shots that apparently killed Markelov and Baburova.
Yermakova said she did not see the actual killing, but she said she heard some shots and then saw a man she identified as Tikhonov run past her in the street.
Yermakova is the second witness to place Tikhonov at the site where Markelov and Baburova were shot.
Khasis told the court earlier that she will testify at the end of the trial.
She also said that Tikhonov was framed by officials "so that the truth about Markelov's murder will be buried once and for all time."
Markelov was involved in defending victims of arbitrary police violence in the North Caucasus. He and Baburova were gunned down in downtown Moscow in broad daylight.
Tikhonov is an active member of an ultranationalist group.
Marches were held in several Russian cities on January 19 to mark the second anniversary of the killings.
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