CAIRO — Under the government of President Abdel Fattah el-Sisi of Egypt, so little dissent is allowed — and what little there is comes at such a high price — that when just a few hundred people across the country called for Mr. el-Sisi’s ouster in a burst of scattered protests on Friday night, it came as a shock.

The apparent trigger for the demonstrations was almost as unexpected: Mohamed Ali, a 45-year-old construction contractor and part-time actor who says he got rich building projects for the Egyptian military and then left for Spain to live in self-imposed exile, where he began posting videos on social media accusing Mr. el-Sisi of corruption and hypocrisy.

In the three weeks since his first video appeared, Mr. Ali has reinvented himself as a whistle-blower, an el-Sisi antagonist and a protest guru, and his tales of corruption at the top have transformed him into a leading voice of opposition to the president. When the protests erupted, it was at the time and date Mr. Ali had urged from afar.

But the extent, and durability, of Mr. Ali’s out-of-nowhere influence — and his ability to spur further demonstrations — remains to be seen. His surge from obscurity to prominence has also raised questions in Egypt about whether his sudden fame has been helped along or exploited by powerful interest groups in the country, inside or outside the government.