German Foreign Minister Heiko Maas has criticised plans to re-run Istanbul's mayoral election as "not transparent and incomprehensible to us".
The re-run on 23 June was ordered by Turkey's election authorities after President Recep Tayyip Erdogan's ruling party complained that the narrow victory of opposition candidate Ekrem Imamoglu was marred by irregularities.
"Who holds the office of Istanbul's mayor can only be decided by the will of the Turkish voters," Mr Maas said in a statement.
"Maintaining democratic principles with transparent electoral conditions is our top priority."
German Foreign Minister Heiko Maas. Source: AAP
The ruling Justice and Development Party (AKP) had complained of "irregularities" and "organised crime" in the Istanbul ballot during nationwide local polls on 31 March.
In a milder reaction than that of Berlin, the French foreign ministry said it was "important that the decision is justified, given the questions that it raises."
Paris urged the Turkish authorities to ensure the "respect for democratic principles, pluralism, fairness, transparency and, in particular, the presence of foreign observers" in the new poll.
The main opposition Republican People's Party (CHP) has condemned the re-run decision by Turkey's top election body as "neither democratic nor legitimate".
However, on Tuesday, Erdogan welcomed a fresh Istanbul election after complaints of corruption as the "best step" for the country.
With an estimated 2.9 million people of either Turkish citizenship or recent ancestry, Germany is home to the largest Turkish diaspora community worldwide.
But the close links between the two countries are also a source of tension, with German politicians regularly blasting illiberal moves by Erdogan.
French President Emmanuel Macron. Source: AAP
French President Emmanuel Macron has tried to forge an amicable working relationship with Erdogan.
But the Turkish leader has lashed out at his French counterpart for designating an annual day to remember the World War I massacres of Armenians, which Paris terms a genocide, a term Ankara vehemently disputes.
The European Union also called for Turkey's election authorities to produce their reasons for ordering the re-run "without delay".
"Ensuring a free, fair and transparent election process is essential to any democracy and is at the heart of the European Union's relations with Turkey," EU diplomatic chief Federica Mogherini said in a statement on Monday.
Erdogan backs re-run of Istanbul vote amid protests
Turkish President Tayyip Erdogan says the decision to re-run Istanbul's mayoral election is an important step towards strengthening democracy, describing the March vote as marred by "organised corruption" and illegality.
The High Election Board ruled on Monday for a fresh Istanbul mayoral contest on 23 June, scrapping the 31 March vote which was lost by Erdogan's party candidate, in a move that drew opposition accusations of dictatorship.
"We see this as an important step to strengthen our democracy," Erdogan told lawmakers from his AK Party in parliament on Tuesday.
"We believe there was organised corruption and full illegality in the Istanbul mayoral elections."
The Turkish lira tumbled against the US dollar after the election decision, bringing its losses this year to 14 per cent as investors fretted about political uncertainty and business leaders criticised the move.
Supporters of Ekrem Imamoglu cheer as they wait for him during a protest against the rerun of elections in Istanbul. Source: EPA
Erdogan said Turkey was facing economic sabotage and vowed to fend off attacks on the economy. "So, what are we going to do? From now on, we will do what we did to terrorists."
Turkey's Western allies have also expressed concern about the election's annulment.
German Foreign Minister Heiko Maas said the decision was neither transparent nor comprehensible.
