* Polls point to narrow victory for "Yes" vote
* Erdogan says greater powers needed to face challenges
* Opponents say Turkey could lurch into authoritarianism
By Tuvan Gumrukcu
ANKARA, April 15 (Reuters) - President Tayyip Erdogan appealed for support from Turkish voters in final campaign rallies on Saturday, the eve of a referendum which could tighten his grip over a country bridging the European Union and a conflict-strewn Middle East.
Opinion polls have given a narrow lead for a "Yes" vote in Sunday's referendum to replace Turkey's parliamentary democracy with an all-powerful presidency, a move Erdogan says is needed to confront the security and political challenges Turkey faces.
Opponents say it is a step towards greater authoritarianism in a country where 40,000 people were arrested and 120,000 sacked or suspended from their jobs in a crackdown following a failed coup attempt against Erdogan last July.
Western countries have criticised that tough response, and relations with the EU - which Turkey has been negotiating to join for a decade - hit a low during the campaign when Erdogan accused European leaders of acting like Nazis for banning referendum rallies in their countries on security grounds.
He has also said Turkey could review a deal under which it limits the flow of migrants - many of them refugees fleeing war in neighbouring Syria and Iraq - into the European Union unless the bloc implements plans to grant Turks visa-free travel.
At a rally in Istanbul, one of four he held in the last hours before Sunday's vote, Erdogan described the constitutional proposals as the biggest change in Turkey since the country was established nearly a century ago, and the culmination of the response to July's abortive putsch.
"Sunday will be a turning point in the fight against terrorist organisations. We will finish what we started on July 15 this April 16," he told a crowd in Istanbul's Tuzla district, decked with Turkish flags and giant pictures of the president.
Erdogan's Islamist-rooted AK Party has enjoyed a disproportionate share of media coverage in the buildup to the vote, but the result may be close. A narrow majority of Turks will vote "Yes", two opinion polls suggested on Thursday, putting his support at only a little over 51 percent.
"Tomorrow is very important, you must absolutely go to the polls," Erdogan urged the crowd. "Don't forget that the vote is our honour."
"TURKEY AT CROSSROADS"
Some 55 million people are eligible to vote at 167,140 polling stations across the nation, which open at 7.00 am (0400 GMT) in the east of the country and close at 5 pm (1400 GMT). Turkish voters abroad have already cast their ballots.