The Europe to Asia bicycle ride was cancelled in June due to the Gezi protests and has now been rescheduled for this Sunday September 22nd. The ride is Turkey's contribution to European Mobility Week and campaigns for cleaner air and safer cycle routes. The ride is sponsored by the UN and EU Delegation to Turkey.
The ride starts at 10:30am so it's probably best to get there a bit earlier I would think. This is the only opportunity you'll get to cross the Bosphorus Bridge on a bike (though runners can do it on foot in the Eurasia run in November).
Meet @ Lutfi Kirdar International Convention & Exhibition Center, Harbiye, nr Taksim | Google Maps link: Protected content
Just turn up with your bike and pedal. No registration required.
More details, flyer and pics (In Turkish) here: Protected content