HONG KONG, June 5 (Reuters) - SpaceX's website and IPO marketing documents were not accessible on Friday in Hong Kong and mainland China, a Reuters review showed, a step that threatens to curb participation by investors there in a listing expected to be the world's largest.

Elon Musk's SpaceX aims to raise $75 billion, the most globally in an IPO, in a deal that would value the company at $1.75 trillion, immediately vaulting it into the ranks of the 10 most valuable U.S.-listed firms.

The company kicked off marketing roadshows on Thursday in New York and the IPO papers were posted on its website, accessible to users in most major Asian markets, except for mainland China and Hong Kong, the Reuters review showed.

Both institutional and retail investors access a company's IPO marketing material to better acquaint themselves with the business and financial details of such candidates, to ensure they pick the best investment prospects.

Reuters could not immediately establish why and since when the website of SpaceX, the rocket, satellite and ​AI company, and the IPO material were restricted in mainland China and the world's No.1 wealth hub of Hong Kong.

SpaceX did not immediately respond to Reuters request for comment outside U.S. working hours.

Spokespersons for the IPO's lead banks, Bank of America, Citigroup, Goldman Sachs, JPMorgan, and Morgan Stanley, did not immediately respond to Reuters requests for comment.

COULD BE FIRST U.S. MARKET DEBUT ABOVE $1 TRILLION

The SpaceX IPO has generated global interest and the listing could become the first U.S. market debut in excess of $1 trillion, which would immediately make it one of the world's most valuable publicly traded companies.

Besides a long list of Wall Street giants, Japan's Mizuho and Macquarie Capital in Australia are involved in managing SpaceX's IPO in Asia Pacific, the roadshow presentation showed.

An "Error 1009" message was displayed in response to attempts to access the company website and marketing material both in mainland China and the global financial centre of Hong Kong, the Reuters review showed.

Web security provider Cloudflare ( NET ) said the most common explanation for the error was that the website owner "has banned" the country or region of the related IP address from access.

Such a block is usually a company decision, said Francis Fong, honorary president of the Hong Kong Information Technology Federation.

While Hong Kong users have also been unable to access some U.S. government websites in recent years, such cases are rare for major companies, he added.

Musk is a household name in China, the world's second-largest economy, where the success of his Tesla electric vehicles makes one of the most popular foreign business figures.

In February, two Democratic U.S. senators urged the Pentagon for an immediate review of SpaceX amid accusations that Chinese investors had secretly acquired stakes in the closely held rocket maker, citing potential national security risks.