German police say they have stopped an Islamist attack, possibly targeting a May Day cycling race attended by thousands every year in Frankfurt.
Officers arrested a couple with suspected militant links, said public prosecutor Albrecht Schreiber.
They found an automatic assault weapon, 100 rounds of live ammunition, chemicals commonly used in preparing home-made bombs and a canister of petrol in their home in Oberursel, near Frankfurt, he said.
"Investigations by the police indicate that we have been able to prevent a terrorist attack," state interior minister Peter Beuth told reporters.
"This incident shows that we must all remain very alert."
The 35-year-old man arrested had dual Turkish-German citizenship and a criminal record, said police. His Turkish wife was also arrested.
Two young children found in their home are being cared for by social services.
Police said they decided to detain the man after he bought large amounts of chemicals, which can be used to make explosives, under a false name.
He was also seen walking along the route of Frankfurt's May Day cycle race, which attracts thousands of participants and spectators every year.
State police chief Stefan Mueller was asked at a news conference in Wiesbaden whether the couple were suspected of planning an attack similar to the 2013 Boston Marathon bombing, which killed three people and injured more than 260 spectators.
He said: "Clearly since the Boston Marathon these security concerns have been part of the considerations of how to deal with that (danger) before every marathon race in Germany - and that is also valid for cycle races."
German newspaper Die Welt quoted security sources saying the couple had links to Salafists in Frankfurt and to the al Qaeda terror network.
Salafists advocate a puritanical form of Islam and Germany's BfV domestic intelligence agency says their numbers are rising, as is the number of potential recruits for Islamic State (IS).
Some 450 people from Germany have travelled to Syria and Iraq to join jihadist forces, BfV estimates.
