* Loss of competitiveness feared by U.S. firms

* Senate Finance panel chairman sounds note of caution

* Deloitte estimates 8 pct boost in corporate tax burden (Adds detail from previous Treasury Dept report, think tank comments, more from Chamber of Commerce, paragraphs 6-7, 20)

By Kim Dixon

WASHINGTON, May 4 (Reuters) - Business groups criticized President Barack Obama's plans to tighten tax rules on overseas activities saying it would make companies less competitive, but U.S. officials called the changes long-overdue fixes to curb abuses.

Obama, carrying out a campaign promise, said on Monday he intends to raise $210 billion over a decade by revising a tax policy that lets U.S.-based global companies defer income earned abroad, and by closing a loophole that administration officials say lets companies hide foreign subsidiaries.

A broad swath of companies -- from technology and pharmaceutical to banking and hedge funds -- would be affected by the changes, experts said.

"It's really hitting most Fortune 100 companies that depend to a great deal on growth of foreign markets for growing their total earnings," said Drew Lyon, a principal at PricewaterhouseCoopers' Washington office who advises Fortune 500 companies on tax policy.

About half of multinationals companies' income is earned abroad, he said.