Violent protests broke out ahead of Wednesday's inauguration of the European Central Bank's new headquarters, with cars set on fire, windows smashed and stones thrown by anti-capitalist activists hours before a massive rally.
Already seven police cars have been set alight and at least two officers have been injured, a police spokeswoman told AFP.
Water cannons have been used to disperse groups of protesters near the security zone erected around the 185-metre (605-foot), 1.3-billion-euro ($1.4-billion) new skyscraper in the east of the city.
Demonstrators were "aggressive and violent," the spokeswoman said.
A number of violent incidents had been reported around the city, centred around the new building itself, but also Frankfurt's venerable Alte Oper concert hall and the financial district, she said.
An AFP journalist at the scene said windows had been smashed at the citizens' centre and burning tyres were in the road nearby.
Convoys of police vans sped through the streets of the financial capital with sirens blaring and helicopters hovered overhead.
The ECB officially inaugurates its headquarters at 11:00 am (1000 GMT) in the presence of central bank president Mario Draghi and around 100 invited guests.
But security is extremely tight with more than 10,000 anti-austerity demonstrators planning a protest rally.
The ECB already moved into the new building situated on the Main river running through Germany's financial capital late last year.
Given the raging Greek debt crisis, the central bank has decided to keep the inaugural celebrations relatively low-key with just 20 guests from outside the institution.
Marching under the Blockupy banner, groups such as Attac, but also unions will join in the rally, alongside representatives from Greece's leftist Syriza party, and Miguel Urban of Spain's Podemos.
A special train of 800 activists is arriving from Berlin and 60 buses from 39 different cities across Europe. Despite the early violence, police said they expected most of the protesters to remain peaceful.
- Massive police presence -
"It is one of the biggest deployments ever in the city," a police spokeswoman told AFP.
At a rally in Frankfurt in March 2012, a policeman was seriously injured when protesters went on the rampage in the city centre, causing around one million euros worth of damage.
At a subsequent demonstration in May 2012, there were around 20,000 marchers, but the rally remained mostly peaceful as more than 5,000 police sealed off large parts of the city.
"We see the European project in mortal danger. Instead of austerity and social cuts, we're calling for more democracy and a fairer balance of power in Europe and within the ECB," a number of unions wrote in a joint statement on Tuesday.
Demonstrators plan to gather in the centre of Frankfurt at 2:00 pm (1300 GMT) and march through the city at 5:00 pm (1600 GMT).
The organisers accused the police and city authorities of "scaremongering" and drawing up an "absurd civil war scenario".
Another spokesman described the massive police deployment as "a scandal".
But Hannah Eberle for the Interventionistische Linke (Interventionist Left) vowed that demonstrators "will not allow the police to get in their way".
"We're fighting for ... a democratic and social Europe," said Roland Suess of the anti-capitalist group Attac.
Until last year, the ECB, which took over monetary policy in the single currency area in 1998, had been housed in a skyscraper, known as the Eurotower, in downtown Frankfurt.
But work began on a new headquarters -- designed by Vienna-based architects Coop Himmelb(l)au -- in 2008.
Incorporating the historic Grossmarkthalle, a wholesale food market, in the east of the city, the building was completed late last year, and the ECB actually moved into the premises shortly afterward.
Against the backdrop of tense talks over a Greek debt deal, the ECB is a prime target of anti-austerity protesters.
Earlier this year, the ECB stopped accepting Greek bonds as security in return for funding, when Greece's new hard-left government under Alexis Tsipras was elected on his promise to scrap the reform-for-aid bailout programme.
Blockupy brings together anti-capitalist and anti-austerity protestors from across Europe.
They camped out in tents at the foot of the ECB's Eurotower headquarters in Frankfurt for months in 2013.