DUBAI, March 20 (Reuters) - Qatar's stock market marginally outperformed Gulf peers in slow early trade on Monday as it completed its upgrade to FTSE Russell's secondary emerging market index, while Saudi Arabia lagged on profit-taking in the insurance sector.

In Qatar, the second phase of the FTSE upgrade took effect from the opening, with 20 of the 22 companies selected in September 2016 seeing their investibility weights doubled.

Stocks in this group were mixed, with Commercial Bank adding 0.9 percent but Qatar Navigation down 0.9 percent. Many investors tried to front-run the passive funds brought in over the past few days by the upgrade.

Saudi Arabia's index was down 0.3 percent in the first half-hour as most insurance shares, which had risen strongly on Sunday, retreated. Solidarity Saudi Takaful, which jumped 3.8 percent on Sunday after it said it had reduced its accumulated losses to below 50 percent of its capital, was down 0.5 percent.

Banking shares were also weak. Heavyweight Al Rajhi lost 2.3 percent as its shares went ex-dividend.

Dubai's index was flat, with builder Drake & Scull, the most heavily traded stock, down 1.1 percent. Emaar Properties was up 0.1 percent.

(Reporting by Celine Aswad; Editing by Andrew Torchia and Catherine Evans) ((celine.aswad@thomsonreuters.com)(+9715 62247653)(Reuters Messaging: celine.aswad.thomsonreuters.com@reuters.net))