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.

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.

The mayor, Ekrem Imamoglu, was detained after prosecutors accused him of corruption and terrorism. The opposition said the arrest was politically motivated.