SARAJEVO (Reuters) - Turkish President Tayyip Erdogan, who will on Sunday address in Sarajevo his supporters from across Europe ahead of a June 24 election, pledged a multi-billion investment in a key motorway connecting Belgrade and Sarajevo.

Slideshow ( 5 images )

Erdogan arrived in the Bosnian capital in what was officially described as a working visit, but the main event will be his speech to thousands of Turks who have come from Western European countries that banned foreign election rallies on their soil.

The transport ministers of Bosnia and Turkey signed a letter of intent for the construction of the highway connecting the two Balkan capitals, a project estimated to cost 3 billion euros ($3.5 billion) but so far blocked by the Bosnian Serbs, who dispute the road route.

Erdogan said he expected the ministries to agree technical details by the summer, when tenders would be called for the construction of the road sections. He added that such a large project would have to involve credits from Turkey’s Exim Bank.

Thousands of Turks have come from Germany, the Netherlands and Austria but also from across the Balkans and Bosnia. Organizers said they expected up to 15,000 people to attend.

“Turkey is our mother nation,” said Coskun Celiloglu, a Macedonian student of Turkish descent. “We came to Sarajevo just for one day to support our saviour Erdogan.”

Police secured the streets and a packed Zetra Olympic hall, where Erdogan’s supporters waved Turkish and Bosnian flags.

“If thousands of people coming from different parts of Europe cannot meet the leader they want to hear, that would not be right for me. Hence we will meet with them here,” Erdogan told a news conference.

A day before his visit, Turkey’s state-run Anadolu news agency reported there had been tip-offs from intelligence agencies about a potential assassination attempt against Erdogan while he visits the Balkans.

Asked about the report, Erdogan said: “This news reached me and indeed that is why I am here,” adding the tip-off was from Turkey’s National Intelligence Agency. “Such threats and operations cannot deter us from this path.”