ISTANBUL — As protests continued for a fourth day in major Turkish cities on Monday, Prime Minister Recep Tayyip Erdogan said the demonstrations were the work of extremists led by political opponents trying to overthrow his government. But Turkey’s president, Abdullah Gul, called for calm from all sides and said protests and demonstrations were a natural part of democracy.

Protesters gathered again in Istanbul and other cities Monday evening, after security forces used tear gas and water cannons overnight on crowds of mostly youthful demonstrators in Istanbul, Ankara and Izmir who were calling for an end to Mr. Erdogan’s more than 10 years in power.

For the first time, deaths were reported at two demonstrations: one protester died in Istanbul after a vehicle slammed into a crowd there late Sunday, The Associated Press reported, citing an Ankara-based medical official. And in the southern border town of Antakya, an opposition party deputy reported on the Web channel Halk TV that a 22-year-old man whom he identified as Abdullah Can Comert was struck in the head by at least one of four bullets fired from an armored police vehicle. Mr. Erdogan accused the main opposition party of using the demonstrations, which flared into a widespread confrontation with security forces on Friday, for political gain.