BERLIN Tue Apr 29, 2014 2:12pm EDT

(Reuters) - Angela Merkel opened her conservatives' European election campaign with an appeal to Germans to use their vote to keep Europe peaceful and strong, reminding them crisis-hit Ukraine will struggle to hold a free presidential election the same day.

"You can go and vote with a completely free choice, on how you see the Europe of the next few years," she told a rally in the northern German city of Bremerhaven.

"People in Ukraine will also vote on May 25. We still need to do a lot of work, to ensure that they can do this at all freely and independently."

In her speech the German chancellor stressed Europe's hard-won peace and freedom of the last years. "As a European Union we insist on Ukrainians' right to chose how they want to live... Russia has no right to meddle in this choice," she said.

"That is why in the next weeks we will continue to seek dialogue, and when necessary, impose sanctions. The difference to the times of a century ago or 75 years ago is that we no longer solve differences with military means. We have ruled this out. We want a peaceful solution to this difficult situation."

A new opinion poll by INSA put Merkel's conservatives on 36 percent, their coalition partners the Social Democrats (SPD) on 28 percent, the opposition Greens on 11 percent and the Left party on 9 percent. The euro-skeptic Alternative for Germany stood at 7 percent.

Hundreds of pro-Russian separatists stormed government buildings across one of Ukraine's provincial capitals on Tuesday and opened fire on police holed up in a regional headquarters, a major escalation of the rebellion in defiance of new Western sanctions.