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UPDATE 6-Oil at highest since mid-2015 on Saudi purge, tighter markets

* Saudi crown prince cements power with high-level arrests

U.S. rig count falls

* Bullish investor positions on Brent hit new record (Updates prices)

By Ahmad Ghaddar

LONDON, Nov 6 (Reuters) - Oil prices hit their highest since July 2015 on Monday as Saudi Arabia's crown prince cemented his power over the weekend with an anti-corruption crackdown, while markets continued to tighten.

Brent crude futures were trading 39 cents higher at $62.46 a barrel by 1339 GMT, after hitting a session peak of $62.90, a 28-month high.

U.S. West Texas Intermediate (WTI) crude rose 20 cents to $55.84 a barrel, having broken above $56 for the first time since July 2015.

Saudi Crown Prince Mohammed bin Salman tightened his grip with the arrest of royals, ministers and investors, including prominent billionaire Alwaleed bin Talal and the powerful head of the National Guard, Prince Miteb bin Abdullah.

Analysts for now do not see Saudi Arabia, the world's largest oil exporter, changing its policy of boosting crude prices.

Prince Mohammed's reforms include a plan to list parts of state-owned oil company Saudi Aramco next year, and a higher oil price is seen as beneficial for its market capitalization.

"We believe the kingdom will stick to the OPEC+ deal and continue to focus on reducing global oil inventories," UBS oil analyst Giovanni Staunovo said.

Saudi Energy Minister Khalid al-Falih said that while there is "satisfaction" with a production-cutting deal between the Organization of the Petroleum Exporting Countries and other producers led by Russia, the "job is not done yet".

OPEC is expected to extend a cut of around 1.8 million barrels per day into the whole of 2018.

"Uncertainty about core regime stability has gone up a bit, so a higher risk premium is justified," Samuel Ciszuk, a senior adviser to the Swedish Energy Agency, told the Reuters Global Oil Forum.

Also boosting oil prices, U.S. energy companies cut eight oil rigs last week, to 729, in the biggest reduction since May 2016.

While supplies are tightening, analysts say demand remains strong.

"Synchronous global economic growth and new supply disruptions are creating the most constructive oil price environment since ... 2014," Barclays bank said.

The bank raised its forecast for the average Brent price in the fourth quarter of this year by $6 to $60 a barrel, and its full-year 2018 forecast by $3 to $55 a barrel.

Speculators have also increased to a record high their bets on gains in the price of Brent.

ICE commitment-of-traders data showed money managers had increased their net long holdings of Brent crude futures and options by 23,500 contracts to 530,237.

Money managers raised their net long commitments on WTI by 63,072 contracts to 343,705 over the same period, a more than six-month high.

(Additional reporting by Roslan Khasawneh and Henning Gloystein in Singapore; Editing by Dale Hudson and David Evans)