Albanian police are investigating a mysterious mass poisoning at an opposition rally at the start of the election campaign which forced dozens of people to seek treatment over the weekend, according to news reports.

The leader of the Democratic Party Lulzim Basha was among more than 140 people affected, regional television N1 reported.

They had taken part in a rally held in Tirana Saturday night. It was still unclear what caused the symptoms - eye and skin irritation and in some cases breathing difficulties.

President Bujar Nishani issued a statement Sunday evening, voicing concern over the health of Basha and others and demanding a full investigation and full disclosure.

The rally marked the start of campaigning for parliamentary elections on June 25.

The polls were initially scheduled for a week earlier, but were postponed in a U.S. and EU-brokered deal between the Democrats and Prime Minister Edi Rama's Socialists. The deal led to the opposition's involvement and oversight of the election process.

The Democrats had previously been boycotting parliament for months and threatened to boycott the elections, too, accusing Rama of tilting the playing field in his favor.