By Bulent Usta and Umit Bektas

ISTANBUL (Reuters) - Turkey's main opposition CHP called on citizens and residents of Istanbul to gather on Sunday, after police set up barricades in areas around its headquarters in the city, in what the party leader called a "siege".

The Republican People's Party (CHP) has been the target of a months-long legal crackdown, which has swept up hundreds of its members - including Istanbul Mayor Ekrem Imamoglu, President Tayyip Erdogan's main political rival - whose March arrest set off Turkey's largest street protests in a decade.

The CHP has denied all accusations against it and said the legal........

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