Nov 12 (Reuters) - Tesla Inc"s (TSLA.O) CEO, Elon Musk, sold another block of company shares worth about $700 million, filings showed on Friday, as the billionaire takes advantage of a meteoric rally that drove the electric-car maker"s value to over $1 trillion. The second round of hefty stock sales this week comes just days after the world"s richest person and Tesla"s top shareholder tweeted that he would sell 10% of his shares if users of the social media platform approved the move. The electric-car maker"s stock was down 3.3% at $1,028.89 on Friday afternoon. Musk"s trust sold 587,638 and 52,099 shares on Thursday in multiple transactions totaling about $687 million, according to two separate filings on Friday. Earlier this week, Musk disclosed he had offloaded some $5.1 billion in Tesla stock. He has now sold about 3% of his shares, according to Reuters calculations. The stock sales, which marked the first time that Musk cashed out on a stake of that size since the company was founded in 2003, were massive by capital market standards, bigger than the initial public offerings of most companies. By getting Twitter users to green-light the move, he has blunted potential criticism of cashing out at a time when Tesla"s valuation has become frothy and shares are at record highs. WINNING STREAK IN DANGER Tesla"s stock fall on Friday put the company"s shares on track to lose about 15% for the week and snap an 11-week winning streak. So far this week, Tesla has lost $180 billion in market value, more than the combined market capitalizations of Ford Motor Co (F.N) and General Motors Co (GM.N). Despite the week"s losses, Tesla is still the most valuable U.S. automaker. Recent strong gains in the stock have underscored demand for shares of electric vehicle (EV) makers. After the blockbuster market debut of Rivian Automotive Inc(RIVN.O) on Wednesday, the two most valuable U.S. automakers are EV companies. In a veiled jab at the Irvine, California-based rival, Musk tweeted on Thursday: "There have been hundreds of automotive startups, both electric & combustion, but Tesla is (the) only American carmaker to reach high volume production & positive cash flow in past 100 years." VOLATILITY Nearly 800,000 options, or about 12% of Tesla"s open contracts, are set to expire at the close of the session on Friday, potentially adding to short-term volatility as traders and options dealers make adjustments. "We expect the share sales will continue, as Musk holds millions of options worth billions of dollars that would otherwise expire worthless, and he has also prearranged share sales under 10b5-1 plans," said Jason Benowitz, senior portfolio manager at the Roosevelt Investment Group LLC in New York. Musk had previously said he would have to exercise a large number of stock options in the next three months, which would create a big tax bill. Selling some of his stock could free up funds to pay the taxes. Prior to the sale, Musk owned a stake of about 23% in Tesla, including stock options. Reporting by Subrat Patnaik, Akash Sriram and Medha Singh in Bengaluru Additional reporting by Caroline Valetkevitch in New York and Hyun Joo Jin in San Francisco Editing by Anil D"Silva, Ira Iosebashvili and Matthew Lewis