One week after the Turkish government arrested the mayor of Istanbul, who is the top rival to President Recep Tayyip Erdogan, the political opposition on Wednesday pressed new tactics to fight what it called an assault on the country’s democracy.

The arrest has set off nightly demonstrations that have drawn hundreds of thousands of antigovernment protesters to the streets of Istanbul and other cities. Now, Mr. Erdogan’s opponents are calling on Turks to boycott companies said to support Mr. Erdogan and vowing to organize a mass protest on Saturday in place of the smaller rallies.

The shift in tactics — including holding public fast-breaking meals for people observing the Muslim holy month of Ramadan — came amid a government crackdown on protesters. The government has issued protest bans in major cities and limited access to social media sites; some protesters have clashed with riot police using water cannons and pepper spray to clear them from the streets.

More than 1,300 people have been arrested in recent days in connection with the demonstrations, the Interior Ministry said, and about 170 have been jailed pending trial. Those arrested included 11 journalists, some of whom remained in custody on Wednesday, including a photographer for Agence France-Presse.

The turmoil began on March 18, when the police arrested Istanbul’s mayor, Ekrem Imamoglu, on accusations of corruption and support for terrorism. On Sunday, he was detained pending a trial on corruption charges and suspended from his post — the same day that his political party designated him as its candidate in the next presidential election.