* Net loss attrib. to shareholders 327 mln yuan, first since 2005
* Firm points to "challenging environment" in marketing for performance
* Senior vice president of search business resigns (Adds comment from CEO Robin Li)
May 17 (Reuters) - Chinese search engine operator Baidu Inc booked its first quarterly loss since at listing in 2005 and forecast quarterly revenue below market estimates, saying a "challenging marketing environment" is sapping income from advertisers.
The tech giant also pointed to the income impact of heightened government scrutiny of online advertising aimed at reducing the visibility of potentially fraudulent businesses.
After the earnings report on Thursday, the price of Baidu's U.S.-listed shares fell as much as 8% in extended trading.
Last year, Baidu said tighter ad regulations as well as an ongoing trade dispute between China and the United States could temper short-term profit, but that investment in new technology would pay off in the long term.
In an earnings briefing, Chief Executive Robin Li said a slowdown in China's technology sector and the economy at large would impact Baidu's performance.
While Beijing is attempting to bolster the economy, "given the current macro conditions, tighter government scrutiny on content, cut backs from the (venture capital) community, and so forth, we are taking a cautious view that online marketing in the near term will face a more challenging environment," Li said.
Baidu reported a net loss attributable to shareholders of 327 million yuan ($47.51 million) for the three months through March 31, versus profit of 6.69 billion yuan in the same period a year prior. The loss was its first since going public in 2005.
Revenue from online marketing services, a key contributor to overall sales, rose nearly 3%, falling slightly short of analyst estimates compiled by researcher FactSet. Total revenue grew 15.4%, matching estimates, according to data from Refinitiv IBES
Excluding one-off items, Baidu earned 2.77 yuan per American depositary share, falling short of analysts' 2.89 estimate.
Baidu forecast second-quarter revenue of 25.1 billion yuan to 26.6 billion yuan. That compared with the 29.30 billion yuan average analyst estimate, showed data from Refinitiv IBES.
Baidu is facing increasing competition in the fast-growing short video and news feed markets, where it has enjoyed an early advantage in data and artificial intelligence as China's biggest search engine provider.
"The pressure from newly emerged (short video) vendor Tik Tok is heating up in 2019, and Baidu must defend itself," said TH Capital analyst Tian X. Hou in a report ahead of the earnings.
Baidu also said Hailong Xiang, senior vice president of its search business, has resigned for "personal reasons". His replacement Dou Shen previously served as vice president of mobile products.
The change could have short-term impact on performance, said Pacific Epoch analyst Raymond Huang.
"Xiang has been with Baidu in the sales force for over 14 years, so when he leaves, you can imagine the internal impact it will have on the whole sales team."
Xiang's departure follows a stream of executive exits, the most notable being that of Chief Operating Officer Lu Qi in May 2018. ($1 = 6.8832 Chinese yuan renminbi)
Reporting by Akanksha Rana in BENGALURU and Josh Horwitz in SHANGHAI; Editing by Anil D'Silva and Christopher Cushing