Who can initiate an optional referendum at the national level?

Art. 125 p. 2 of the Constitution, art. 60, art. 61, art. 63 of the Law on Nationwide Referendum

Constitution of the Republic of Poland adopted on 2April 1997

http://www.sejm.gov.pl/prawo/konst/angielski/kon1.htm (in English - the Parliament’s website)

Law on Nationwide Referendum adopted on 14 March 2003

http://pkw.gov.pl/g2/i/00/8.pdf (in English - but not updated)

http://isap.sejm.gov.pl/DetailsServlet?id=WDU20030570507 (in Polish – the Parliament’s website)

Theright to order a nationwide referendum is vested in the Sejm, who can call thereferendum by an absolute majority of votes in the presence of at least half ofthe statutory number of Deputies, or in the President of the Republic with theconsent of the Senate given by an absolute majority vote taken in the presenceof at least half of the statutory number of Senators.

Sejmmay decide to call referendum in particular case on its own initiative or atthe request of the Senate, the Council of Ministers or group of at least 500 000citizens.