Legal provisions for mandatory referendums at national level

Poland
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)