Oksana Grigorieva said the concert proceeds will pay for vital surgery for children born in the area affected by radioactivity from the 1986 nuclear plant explosion and fire.
Grigorieva told journalists she already has funded operations for 50 children with heart conditions and leukemia in Ukraine, Belarus and Russia.
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"Children’s pain hurts me too," she told a news conference. "I always wanted to help children through music."
Grigorieva and Gibson split this month after a three-year romance. She said they had agreed to raise their baby daughter Lucia together.
She denied media reports that Gibson asked for a DNA test to prove his fatherhood.
"It’s not true," she said. "There has been a lot of lies written about us."
She also said she would remove a hammer-and-sickle tattoo on her ankle, which Gibson had advised her to get.
After announcing his relation with Grigorieva last year, Gibson filed for a divorce with his wife Robyn after 28 years of marriage and seven children together.
Grigorieva also has a son with actor Timothy Dalton.
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