One of the suspects was seen along the route of a popular May Day Rund um den Finanzplatz Eschborn, which was won by Katusha's Alexander Kristoff last year.
This year's event has been cancelled as a result of the incident, and could lead to security being heightened at the upcoming European Grand Tours - the Giro d'Italia this month and the Tour de France in July.
Public prosecutor Albrecht Schreiber said a search of the couple's home had turned up an automatic assault weapon, 100 rounds of live ammunition, chemicals commonly used in preparing homemade bombs and a canister full of petrol.
"Investigations by the police indicate that we have been able to prevent a terrorist attack," Peter Beuth, the interior minister of Hesse, told reporters. "This incident shows that we must all remain very alert."
The 35-year-old male suspect had dual Turkish-German citizenship and a criminal record, said Hesse police chief Stefan Mueller. His wife is Turkish and two young children found in their home by police were being cared for by social services.
Police decided to detain the man after he was seen buying large amounts of chemicals which can be used to make explosives, using a false name. He was also seen along the route of the Frankfurt race, which attracts thousands of participants and spectators every year.
Asked if he suspected the pair had been preparing an attack similar to the 2013 Boston Marathon bombing a year ago, which killed three people and injured more than 260 spectators, Mueller 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 identified the couple by their first names and initial - Halil and Senay D. - and said they had links to the Salafist scene in Frankfurt and to the al Qaeda network. It did not identify the source for its report.
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.
The BfV estimates that 450 people from Germany have travelled to Syria and Iraq to join radical jihadist forces.