As yet, there is no information on President Boris Tadic's whereabouts. According to eyewitnesses, the suspect has pulled the safety pin out of one of the grenades.
Police have cordoned off the area and are negotiating with the suspect.
The incident took place in the wake of U.S. Vice President Joseph Biden's visit to Serbia, where he met with President Tadic for talks.
A number of Serbian political opposition parties and nationalist organizations protested against Biden's visit, citing the NATO bombing of the former Yugoslavia in 1999 and U.S. support for Kosovo, which unilaterally declared its independence from Serbia last year.