Two years of hard work to build international investor confidence in Turkey's economic turnaround story has been dealt a major setback by a long-feared detention of President Tayyip Erdogan's main political rival.

As the detention of Istanbul's mayor and the arrest of more than 100 raise alarms over an escalating crackdown on opposition, internet access across Turkey has slowed and protests are banned in Istanbul.

Turkish police have detained the strongest rival to President Recep Tayyip Erdogan, a move that experts say is aimed at removing all possible contenders ahead of the next presidential election and further stretching his rule.