(Adds quote, human rights)
DUBAI, Jan 3 (Reuters) - Iranian Supreme Leader Ayatollah Ali Khamenei, attacking Saudi Arabia for the second straight day over its execution of a prominent Shi'ite cleric, said on Sunday politicians in the Sunni kingdom would face divine retribution for his death.
"The unjustly spilled blood of this oppressed martyr will no doubt soon show its effect and divine vengeance will befall Saudi politicians," state TV reported Khamenei as saying. It said he described the execution as a "political error".
Saudi Arabia executed Nimr al-Nimr and three other Shi'ites alongside dozens of al Qaeda members on Saturday, signalling it would not tolerate attacks by either Sunni jihadists or members of the Shi'ite minority seeking equality.
Khamenei added: "This oppressed cleric did not encourage people to join an armed movement, nor did he engage in secret plotting, and he only voiced public criticism ... based on religious fervour."
In an apparent swipe at Saudi Arabia's Western allies, Khamenei criticised "the silence of the supposed backers of freedom, democracy and human rights" over the execution.
"Why are those who claim to support human rights quiet? Why do those who claim to back freedom and democracy support this (Saudi) government?" Khamenei was quoted as saying.
While Western human rights groups have condemned the executions, Western government responses have so far been muted.
The U.S. State Department expressed concern that Nimr's execution could exacerbate sectarian tensions in the Middle East. In Hawaii, where President Barack Obama is on vacation with his family, White House deputy national security adviser Ben Rhodes said the administration has urged the Saudis to show restraint regarding respect for human rights.
(Reporting by Dubai newsroom; Writing by William Maclean; Editing by Paul Tait and William Hardy)