Turkey has reportedly deployed some 25,000 police on the first anniversary of Istanbul's Taksim Square protests.
Security forces in riot gear have blocked access to the square.
PM Recep Tayyip Erdogan earlier urged youths to ignore calls for nationwide rallies to mark last year's deadly crackdown on demonstrators.
Protests against plans to redevelop Istanbul's Gezi Park turned into mass anti-government rallies after a heavy-handed police response.
A number of people were killed in the unrest, with thousands more injured.
A 64-year-old woman, who fell into a coma after inhaling tear gas during another crackdown on protesters in the capital in December, died on Friday.
The BBC's James Reynolds, in Istanbul, says there is heavy police presence in the city centre, with many plainclothes officers patrolling the streets.
A government ban on gatherings is currently in force in Taksim Square, where water cannon and armoured vehicles have been deployed.
A number of protesters were reportedly arrested as they tried to march on the square on Saturday.
Security forces kicked and detained a CNN reporter during a live broadcast from Taksim.
Ivan Watson said in a Twitter post that he and his film crew were "released after half an hour".
Riot police have also blocked access to nearby Gezi Park amid fears of clashes, Turkish media report.
The main organisers behind last year's Gezi Park protests - Taksim Solidarity - have called for a demonstration to mark the one-year anniversary.
Rallies were also expected to take place in several other Turkish cities, including Ankara, Izmir and Antakya.
But Mr Erdogan warned young people not to take part, saying: "One year later, people, including so-called artists, are calling for demonstrations, but you, Turkey's youth, you will not respond to the call."
Addressing a crowd of young people in the capital on Friday, he described the movement as "terrorist organisations" that "manipulated our morally and financially weak youth to attack our unity and put our economy under threat".