Around 100 events are being organised across the UK to remember the thousands of Muslim men and boys from Srebrenica who were killed by Bosnian Serb forces in 1995.
According to the BBC, UK Prime Minister David Cameron has led tributes ahead of the Westminster Abbey service.
Cameron said "must never, ever forget what happened".
The massacre was the worst event in Europe since World War Two.
More than 2,000 invited guests at Monday's service in Westminster Abbey will witness candles being lit in memory of those killed.
