* European watchdog says criticism of flood policy censored
* Prime Minister Vucic says will fight "lies"
* Ombudsman warns of organised bid to stifle criticism
BELGRADE, June 2 (Reuters) - Serbia's prime minister demanded an apology from Europe's chief security and rights watchdog on Monday after it accused his government of trying to smother online criticism of its handling of devastating floods last month.
Dunja Mijatovic, freedom of media representative for the Organisation for Security and Cooperation in Europe (OSCE), said in a statement last week she was deeply concerned about what she described as a "worrying trend of online censorship in Serbia".
She cited the removal of online content perceived as critical of Belgrade's flood response and the detention of three people for allegedly spreading panic through online posts.
Some 51 people died in Serbia and thousands more lost their homes in May when the heaviest rainfall recorded in more than a century caused rivers to burst their banks, submerging towns and washing away roads and bridges.
Prime Minister Aleksandar Vucic said he was "deeply worried" by Mijatovic's allegations and accused the OSCE of waging the "dirtiest campaign" against him and Serbia. He challenged the group to produce evidence to support its accusations.
Vucic is a former ultranationalist who served as information minister in the late 1990s, when independent media were fined and shuttered under draconian legislation designed to silence dissent as strongman Slobodan Milosevic readied for war with NATO over Kosovo.
Vucic changed tack to embrace Serbia's path to European Union membership in 2008 and returned to government in 2012.
"From you, dear gentlemen from the OSCE, I expect a simple apology and nothing more," Vucic wrote. "Whether or not I get it, I want to inform you, dear gentlemen, that I will fight the lies you have spread."
Vucic's strong grip on power has unnerved some in Serbia who fear a return to authoritarianism, something the prime minister has dismissed. Much of that fear has been channelled through blog posts and online media.
Using powers under a state of emergency declared during the flooding, police took three people in for questioning on suspicion of "inciting panic" by their online posts, and summoned about a dozen more.
Serbia's human rights ombudsman warned on Monday of an "emergency situation" in terms of freedom of expression.
"The sequence of events ... suggests that this is not a question of someone's poor understanding of the situation, but an organised effort aimed at stifling criticism," Sasa Janovic told Serbia's B92 television. (Editing by Matt Robinson/Mark Heinrich)