Aug 17 (Reuters) - Online trading platform Plus500 forecast annual revenue to be "significantly ahead" of analysts' estimates and reported a lower first-half profit on Tuesday as trading levels normalised from record volumes in 2020.

The London-listed firm expects revenue for the full year ending Dec. 31, 2021 to be ahead of a company-compiled analysts' estimate of $476.7 million. It had reported 2020 annual revenue of $872.5 million and $354.5 million in the year earlier.

Trading platforms, which were boosted by unprecedented market volatility in the first half of 2020 when the COVID-19 pandemic's onset spurred a global sell-off, have milked gains this year from a retail frenzy around the so-called meme stocks.

"The unique market environment, which emerged during early 2020, and the sustained levels of volatility that it produced across global markets, has driven an unparalleled number of opportunities for customers to trade," the company said in a statement.

Plus500, which rivals IG and CMC Markets, also announced a new share buyback programme of $12.6 million and an interim dividend worth $60 million, down from $101 million declared the year earlier.

Customer additions, it said, stood at 136,980 during the first half, down from 198,176 in the year-ago period.

Core profit for the company, which launched a share dealing platform earlier in the year, was $187.6 million for the six months ended June 30, compared with $361.8 million a year earlier.

(Reporting by Aby Jose Koilparambil in Bengaluru; editing by Uttaresh.V)