MONROVIA (Reuters) - Liberia’s ruling party, whose candidate finished runner-up in the first round of this month’s presidential election, said on Sunday it would back a legal challenge to the result, accusing President Ellen Johnson Sirleaf of interfering in the vote.

The extraordinary charge by Unity Party against Johnson Sirleaf, one of its own members, throws into question a second round run-off scheduled for Nov. 7 between its candidate Vice President Joseph Boakai and front-runner George Weah.

Unity Party said in a statement that the Oct. 10 poll, meant to usher in Liberia’s first democratic transition of power since 1944, was “characterised by massive systematic irregularities and fraud”.

The statement, read to reporters by Unity Party Chairman Wilmont Paye, said Johnson Sirleaf had acted inappropriately by meeting privately with elections magistrates before the vote.

“Greed has resurfaced in its most callous form, sowing seeds of discord yet again with the intent of disrupting the fragile peace of Liberia,” the party said in its statement.

Unity Party officials said they were supporting a legal challenge by Liberty Party of the third-place candidate Charles Brumskine, which has petitioned the elections commission for a re-run of the first round. The All Liberian Party of businessman Benoni Urey also said on Sunday it was backing the complaint.

“It doesn’t mean we will not take part in the (run-off),” Augustine Ngafuan, Unity Party’s national campaign chairman, told Reuters. “We hope the court can rule before the run-off. If not, we will decide what next to do.”

An official from the elections commission said it expected to hear the formal complaint on Monday. The commission’s findings can be appealed in the Supreme Court.

At Johnson Sirleaf’s residence, where she was celebrating her 79th birthday, her spokesman Jerolinmek Piah told reporters that he would comment on the accusations later as he did not want to overshadow the festivities.

International observers from the European Union, the Carter Center and the National Democratic Institute have said they saw no major problems with the vote.

Unity Party’s statement cements a falling out between Johnson Sirleaf, a Nobel Peace Prize laureate, and her party’s leadership after 12 years in power that saw the country consolidate a post-war peace but it also drew sharp criticism over alleged corruption and underdevelopment.

Boakai has served as Johnson Sirleaf’s vice president since the president’s inauguration in 2006. However, Johnson Sirleaf declined to endorse him and he distanced himself from the last administration.

Weah, a former soccer star in Europe, won the first round with 38.4 percent of the vote to Boakai’s 28.8 percent and has momentum heading into the run-off. On Thursday he picked up the endorsement of former warlord Prince Johnson, who won 8 percent of the vote in the first round.

Morluba Morlu, a senior official from Weah’s CDC party, said those challenging the result were “opting to stage trouble”.

“But no matter what they do ... they cannot stop the election of Ambassador Weah,” he added. Weah has been a UNICEF goodwill ambassador.