* PM Yildirim launches campaign ahead of April referendum
* Constitutional reform to broaden Erdogan's powers
* Erdogan supporters say change will ensure stability
* Opponents fear surge in authoritarianism (Adds Yildirim comments, background)
By Ercan Gurses
ANKARA, Feb 25 (Reuters) - Turkey's prime minister called on voters on Saturday to back a change to the constitution in an April referendum that would greatly strengthen President Tayyip Erdogan's powers, portraying the reform as a vote for stability.
Thousands of ruling AK Party members waved Turkish flags and rallying songs resounded around a packed sports arena in the capital as Prime Minister Binali Yildirim handed out red carnations and kicked off the campaign for a Yes vote in April.
"There is a rising voice from the town squares: for a strong Turkey, for lasting stability our decision is ..." Yildirim called out earlier to thousands who couldn't get into the full stadium, eliciting the shouted response "Yes".
The bid to replace the EU candidate country's parliamentary democracy with a powerful executive presidency is seen by Erdogan supporters as a guarantee of stability, preventing a return to the fragile coalitions of previous decades.
But opponents fear a surge in authoritarianism.
Tens of thousands of people have already been detained and more than 100,000 public sector workers suspended or dismissed since an attempted coup last July.
The NATO member of 80 million people will vote on the reform in a plebiscite on April 16, with a simple majority needed to approve legislation passed by parliament in January and rubber-stamped by Erdogan last month.
Erdogan says an overhauled political system is needed to tackle unprecedented security threats, from a series of Islamic State and Kurdish militant bombings to the putsch during which rogue soldiers killed at least 240 people.
"The new system will bring the end of terror, it will destroy terror," Yildirim said, saying Kurdish militants, Islamic State and supporters of the U.S.-based cleric accused of orchestrating the failed coup all sought a "No" vote.
PM REJECTS POLARISATION
Erdogan has said voting against the change means siding with supporters of terrorism, but Yildirim sought to assuage concerns about a divisive campaign, saying the AKP aimed to unite rather than polarise and would reach out to opposition supporters. The nationalist MHP opposition supports the reform.
Erdogan, the most popular but also most divisive politician in modern Turkish history, has long cast himself as the champion of ordinary, pious Turks exploited by a secular elite. But some pollsters and people close to the AK Party now think his polarising rhetoric risks scaring off moderate voters in April.