ISTANBUL — Even before President Recep Tayyip Erdogan of Turkey was inaugurated last week, he began elbowing his way into the front ranks of the globe’s strongmen.

Hours before taking the oath of office — after 15 years already in power — Mr. Erdogan published a 143-page decree changing the way almost every government department and public body in the country operates.

In the days since, he has issued several equally lengthy decrees and presidential decisions, centralizing power and giving him the ability to exert control in nearly all areas of life with almost unchecked authority.

At a moment when democratic systems around the world are under increasing pressure, Mr. Erdogan, who was re-elected in June, is among those leaders, like Vladimir V. Putin of Russia and Viktor Orban of Hungary, who are using the levers of democracy to vastly expand their authority.