* Berlin, Kiev say high participation crucial on May 25
* About 1,000 OSCE observers will follow election (Adds quotes and details)
BERLIN, May 20 Ukraine's presidential election on Sunday can help bring stability, Germany's foreign minister said on Tuesday, but it is crucial that as many people as possible vote to reinforce its democratic legitimacy.
The days left to the vote must be used to ensure polling stations can open and people can vote free from intimidation, Frank-Walter Steinmeier told a news conference in Berlin, but he warned the situation in parts of the east remained fragile.
"This presidential election cannot be overestimated," Steinmeier said. "It is a decisive marker, a real chance for a new start for Ukraine and it could be the key to regaining economic and political stability."
A third set of talks between Kiev politicians and regional groups aimed at defusing tensions will take place in the southeastern city of Mykolayiv beforehand, Ukraine's acting foreign minister, Andriy Deshchytsia, said in Berlin.
Kiev's interim authorities hope the election will restore calm after five months of upheaval that has included violent street protests, Russia's annexation of Crimea and pro-Russian separatist rebellions in the east of the former Soviet republic.
"There are still serious risks ... in some areas of eastern Ukraine, separatists want to block the election and in some areas the security situation, even if it has eased a little in the last days, remains very fragile," the German minister said.
Around 1,000 election observers from the Organisation for Security and Co-operation in Europe (OSCE) will follow the event, Steinmeier said.
"The situation in the east remains unstable. Extremist groups active there are not only creating chaos and terrorising the population, they are also preventing a calm preparation of the election," Deshchytsia said.
"Unfortunately, we have only limited control over some areas, but we hope that those people who want to take part in the elections will get the chance to exercise their right to vote."
Asked if there was a turnout threshold necessary for legitimacy, Steinmeier said: "You cannot dictate the level of election participation in advance. It is about giving as many people as possible the chance to vote, and ensuring that these are free, general elections."
According to a poll on Tuesday, confectionary tycoon Petro Poroshenko, who backed the pro-European uprising that ousted Moscow-backed president Viktor Yanukovich in February, would get support from 53.2 percent of voters in the first round.
Former prime minister Yulia Tymoshenko remained in distant second place on 10.1 percent.
If no candidate obtains more than 50 percent in the first round of voting on May 25, there will be a run-off vote between the two leading contenders on June 15.
President Vladimir Putin has said Sunday's election could be "a step in the right direction" but other Russian officials have signalled that Moscow may not recognise the outcome, especially if Kiev continues to use its armed forces in eastern Ukraine. (Writing by Alexandra Hudson and Michael Nienaber; Editing by Stephen Brown, Larry King)