Flame acceleration (FA) and the deflagration-to-detonation transition (DDT) are among the most interesting and exciting phenomena in the field of combustion and explosion of gases. From both practical and theoretical points of view, it is important to understand the basic laws governing these phenomena as well as the physical and/or chemical mechanisms and features of the process. High-speed flame-front photography during the deflagration of a premixed gas mixture in a long smooth tube with transparent walls was performed. A stoichiometric mixture of acetylene with oxygen diluted with argon by 25% is used. The experiments are carried out in a transparent cylindrical tube with an inner diameter of 60 mm and a length of 6 meters. The evolution of the structure and shape of the flame front from the moment of initiation of deflagration by a weak ignition source to the formation of a detonation wave is determined. Four characteristic phases of the propagation process are distinguished: at the first stage, the flame accelerates, then slows down, followed by flame propagation at an almost constant speed, and finally repeated acceleration, during which detonation is formed. It is shown how the dynamics of the process changes with a change in the initial pressure of the mixture. The most interesting and poorly studied stage of the DDT, the stage of intensive reacceleration, during which the flame abruptly changes shape, is described in detail.