A sleeper cell acting on orders from the Islamic State (IS) group carried out the massive bombings on a peace rally in Ankara this month to try to disrupt Turkey's election, prosecutors said Wednesday.

The Ankara prosecutor's office said there was "strong evidence" that the cell was also behind other attacks in the country, including one on the border with Syria in July, and was planning more atrocities.

Riot police firing tear gas and water cannon on Wednesday stormed the Istanbul headquarters of a media group linked to President Recep Tayyip Erdogan's bitter rival, on what was described as a dark day for democracy ahead of a pivotal election.

The action triggered widespread concern about media freedom in Turkey, with critics accusing the government of trying to silence opponents of Erdogan's Justice and Development Party (AKP) before Sunday's legislative vote.

Selahattin Demirtas shared a cup of tea at dawn on Sunday with a few workers on a Turkish building site, joined students for a simple omelet at noon and then made a speech to just a few hundred party members in a dimly-lit Istanbul conference room.

While his rivals stage colorful mass rallies, where the flags fly and the rhetoric flows, the leader of Turkey's main pro-Kurdish party has been forced into running a very low-key campaign for the weekend election.

Turkey pressed on with its "war on terror" on Tuesday, just five days before the country goes to the polls, detaining dozens of Islamic State suspects and hitting Kurdish rebels across the border in Syria.

President Recep Tayyip Erdogan has been playing the security card in the run-up to Sunday's vote with tensions running high over the renewed Turkish conflict and a massive bomb attack blamed on IS jihadists.

President Barack Obama will travel to Turkey for a G20 summit, followed by stops in the Philippines and Malaysia for meetings with world leaders next month, the White House said Monday.

At the G20 summit, leaders from the world's major economies are due to give final approval to a new plan to crack down on tax evasion by multinational corporations that costs countries at least $100 billion a year.

Kurdish fighters in northern Syria have accused the Turkish military of attacking their positions near a border town at the weekend.

There was no immediate confirmation from Turkey, where an official said the government was investigating the claims.

Seven suspected Islamic State militants and two Turkish police officers were killed on Monday in a fierce gun battle in the main city in the Kurdish-majority southeast, security sources said.

The shootout in Diyarbakir erupted just six days ahead of elections in Turkey, with tensions running high following the country's worst ever bomb attack and a resurgence of the conflict with Kurdistan Workers' Party (PKK) rebels.

A week ahead of Turkey's second election in five months, President Recep Tayyip Erdogan's party is working overtime to try to reclaim its parliamentary majority, in a climate of tension fueled by the Ankara attacks and the reignited Kurdish conflict.

Prime Minister Ahmet Davutoglu, leader of the dominant Justice and Development Party (AKP), is holding a mass campaign rally in Istanbul on Sunday, hoping to drum up enough support to defy the opinion polls that predict a replay of the June vote.

At least 12 people were injured in a scuffle between Turks and ethnic Kurds as hundreds of Turkish citizens gathered Sunday at their embassy in Tokyo to vote in a general election, reports said.

TV footage showed police breaking up the fight outside the mission as voting for the November 1 election began early in the Japanese capital.

Prosecutors in Turkey have opened a preliminary investigation into the country's interior minister, accused of negligence after the Ankara bombing that killed 102 people this month, the Hurriyet newspaper said Friday.

The probe is unlikely to get off the ground, however, because prosecutors would need a green light from the government to strip minister Selami Altinok of his political immunity and launch a full investigation.