By Andrea Shalal and Ece Toksabay

BERLIN/ANKARA (Reuters) - German officials on Thursday blocked events where two senior Turkish government ministers planned to speak amid growing public outrage over Ankara's arrest of a Turkish-German journalist, dragging bilateral ties to a new low.

Turkish Justice Minister Bekir Bozdag said he had called off a meeting with his counterpart in Germany and was returning home after a talk he was due to give in the southwestern town Gaggenau was canceled, with officials citing concerns about the size of the venue.

The city of Cologne also blocked an event where Turkish Economy Minister Nihat Zeybecki was to speak on Sunday, amid security concerns.

In Ankara, the Turkish foreign ministry summoned the German ambassador for an explanation, ministry sources said.

The canceled events follow Turkey's arrest of Deniz Yucel, a correspondent for the prominent Die Welt newspaper, on Monday.

German Foreign Minister Sigmar Gabriel told reporters in Kiev that the arrest had caused grave damage to ties between the two NATO allies and added that foreign ministry officials had spoken with Turkey's ambassador twice this week.

"We must solve this problem as soon as possible; the damage that currently exists is extraordinarily large," Gabriel said.

Yucel, a citizen of both Germany and Turkey, faces up to 10-1/2 years in jail if convicted of charges of propaganda in support of a terrorist organization and inciting public violence. He denies the charges.

He is the first German journalist held in a widespread crackdown in Turkey that has often targeted the media following a failed coup last July.

The arrest has soured already-strained ties between the two countries, who are at odds over the post-coup crackdown and a German investigation into possible spying by Turkish clerics.

Chancellor Angela Merkel on Wednesday underscored the importance of independent journalism and said her government would "do everything in its power" to secure Yucel's release.

Officials in Gaggenau withdrew permission for an event hosted by the Union of European-Turkish Democrats (UETD) where Bozdag was due to speak, saying the venue was too small for the expected crowd.

Bozdag was due to speak to the supporters of Turkish President Tayyip Erdogan as part of Ankara's effort to rally support among Germany's estimated 1.5 million Turkish citizens for an April referendum on expanding the powers of the Turkish presidency.

Erdogan also reportedly plans a visit, although the German foreign ministry said it had not been notified.

Bozdag told reporters he would return to Turkey instead of meeting with his German counterpart.

A German government official said the justice ministry had worked intensively to arrange a meeting. "We wanted to leave no stone unturned to secure a fair process for Deniz Yucel," the official said.

The city of Cologne also cited "security concerns and personnel constraints" in blocking an event where Zeybecki was to speak on Sunday.

Spokesman Gregor Timmer said the city felt deceived because the room was initially reserved for a theater production. He added that the local UETD group only notified the city of its plan to hold a meeting about the referendum on Wednesday.

He said no contract had been signed.

(Reporting by Andrea Shalal; Editing by Madeline Chambers and Toby Davis)