[JURIST] Detainees at prisons across Iraq who have not yet been tried cast their votes in the country's constitutional referendum [JURIST news archive] Thursday as authorities stepped up security [AP report] in preparation for the full referendum on Saturday. Those casting votes included detainees at Abu Ghraib [JURIST news archive] prison and Camp Bucca as authorities sent ballot boxes to the prisons. The Iraqi Electoral Commission [official website] had announced previously that all unconvicted prisoners in the country would be eligible to vote [JURIST report] in the referendum. It was not known whether Saddam Hussein [JURIST news archive] had voted, although authorities had previously said he would be allowed. Cities around the country were unusually quiet on Thursday as authorities began to heighten security and the country began a four-day national holiday leading up to the referendum on Saturday. Curfews took effect Thursday, the country's borders will close Friday and all travel among provinces will be barred on Saturday as the US and Iraq try to halt insurgent attacks designed to suppress turnout for the vote. The country's most influential Shiite cleric, Grand Ayatollah ali al-Sistani [official website], on Thursday reiterated his earlier support [JURIST report] for the constitution and urged Iraqis to approve [Independent report] the document. AP has more.