In the Southwestern corner of the area with La Touche, you will find the a covered stall. This will allow Arno to get the Crowd more riled up and providing a bigger distraction that he can use to his advantage.
From the starting point, head to the right, West. Keep to the outer part of the rooftops. You will avoid most of the Snipers watching the area that way. One of the houses has a useful passage though. On the Eastern side you can slip into the house. On the Western side, take cover under the window and wait for the nearby Sniper. Take him out then head back through the window. Drop down to ground level and go through the house. This will give you the set up for another cover kill. From there you can start toward the produce cart.
Alternatively, you can stay on the rooftop. Watch the movements of the guard below. When any one is singled out, use the Phantom Blade to quickly kill them with a headshot. Be sure to also look down. Below you, behind the fence nearby the food cart is another guard. A quick headshot or some careful use of the Last Known Position can get you either a cover kill or a headshot kill.
Either way, watch for the guard watching the cart to move away. Take cover and then take him out when he gets close to get another cover kill and to give you a safe opportunity to trigger the crowd assistance.