Some 61.5 million voters can cast their ballots and decide the composition of the new Bundestag.
The biggest party will decide the new chancellor and polls predict that will be the CDU, giving Merkel a fourth term in office.
Undeterred by the weather, Berliners lined up at polling stations, as 40,000-plus participants in the Berlin Marathon ran through the capital's streets.
“I am going to vote after watching the marathon,” says Kurt Wascher. “But I still have to find out where my voting bureau is. The weather isn’t exactly inviting, but what can you do?”
Not everybody is happy with the marathon. “It doesn’t work together,” says Frido Kailing who said he voted for the Liberal FDP party. “These are two different things. The one is noise and fun, voting is a serious thing.”
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Voters at polling stations near the route of the marathon have to walk up to a kilometre to find a place to cross the road, often by going into a U-bahn station.
In total there are 1,779 polling stations in Berlin. Most of them are in schools, homes for the elderly or community club premises.
But there are more unusual sites - a bowling club, a swimming pool and even the canteen of a mint liquor brewery.
French culture and German voting
One of the most peculiar locations is the voting bureau at 211 Kurfürster Damm, which is on the second floor of the French cultural centre, the Maison de France.
“Fantastic!" says Fredrick Slechlin, on coming out of the building with his three-year-old son. “I even wrote to my friends on social media. It is really a special feeling voting in this place because there is nothing more European than to vote for a German election here in the Institut Français. I appreciate it a lot."
Voters who come before 3.00pm even have a chance to see a screening of the Hungarian movie Teströl és lélekröl (On Body and Soul) by Ildikó Enyedi that won the Golden Bear at the Berlin Film Festival.
Slechlin is not concerned by the marathon. ”Of course it is always a little bit complicated here having so many events, especially here in Berlin but we are used to it and know how to get along with it."
He voted for the Martin Shcultz's Social Democrats (SPD), mainly because they call for better child care. “It’s because of my son. I am not really related to one party,” he said but he added that he voted for one of the establishment parties because he is worried by the rise of the far right.
“On the one hand I say, OK, we are in a democratic society, we have get along with people who sing in different ways," he comments. "But I’m a little bit worried because [they are] just doing politics with hate. It’s not solution-driven politics.”
Kenyan Eliud Kipchoge finished the marathon first on 2h03'32", only 35 under the world record. First of the women was fellow Kenyan athlete Gladys Cherono at 2h17'21".
Polls close at 6.00pm.