CHISINAU (Reuters) - Moldova's Socialists, the ex-Soviet state's largest opposition party, put forward a dismissed prosecutor general as a "unified opposition" candidate on Monday to challenge pro-European incumbent Maia Sandu in an October presidential election.

Sandu, who has made securing European Union membership the cornerstone of her policies, is seeking a second mandate in a poll held alongside a referendum on joining the 27-nation bloc.

The pro-Russia Socialists say they do not oppose the campaign for European integration, but accuse Sandu of using the referendum as a vehicle to win the election.

Other Moscow-friendly groups are openly campaigning against the plebiscite in Moldova, one of Europe's poorest countries lying between Ukraine and Romania.

Former prosecutor Alexandr Stoianoglo, the standardbearer endorsed by the Socialists, told a news conference he was running to end injustices under Sandu's administration.

"The constitutional order is being violated every day by the authorities and various political scoundrels," he said.

Stoianoglo said he favoured EU membership, "but I find it insulting that European integration is used as a truncheon and not as a means to modernise the country and unite its people. A European Moldova must be neutral to leave room for cooperation with other countries and peoples".

Sandu sacked Stoianoglo last year, citing violations of the criminal code on exceeding his authority and a failure to tackle entrenched corruption.

He denies wrongdoing and has yet to be convicted in a court. The European Court for Human Rights has ruled that his dismissal violated principles of due process.

Socialist Party leader Igor Dodon, a former president defeated by Sandu in 2020, said the party was backing Stoianoglo as a "test of political maturity."

"Society expects clarity and a common strategy from its politicians," Dodon said.

There was no indication whether Moldova's other major opposition grouping, led by fugitive business magnate Ilan Shor, would back Stoianoglo.

Shor, sentenced in absentia to 15 years in prison last year for mass fraud, lives in Russia and heads the "Victory" electoral bloc made up of several pro-Moscow groups.

Dodon trails the president in opinion polls, which also suggest 56% of voters will say "yes" to joining the EU. The bloc last month opened membership talks with both Moldova and Ukraine.

The United States, Britain and Canada last month accused Russia of plotting to sway the outcome of the presidential vote and incite protests if a pro-Moscow candidate should lose.

(Reporting by Alexander Tanas, Editing by Ron Popeski and Bill Berkrot)

By Alexander Tanas