* Protests break out in Cairo and other cities
* Stocks slip further after plunging on Sunday
* (Updates number of arrests)
CAIRO, Sept 23 (Reuters) - Egyptian authorities rounded up more than 400 people in response to an outbreak of protests against President Abdel Fattah al-Sisi over the weekend and security forces stepped up their presence in central Cairo, human rights monitors said on Monday.
In a rare pubic display of dissent, hundreds took to the streets in the capital and other cities on Friday in response to calls for protests against alleged government corruption. The protests spread to the Red Sea city of Suez on Saturday, residents said.
The demonstrators, shouting slogans calling on Sisi to give up power, were defying a ban on protests.
Gamal Eid, director of the Arab Network for Human Rights Information, said more than 400 people in different parts of the country have been arrested in connection with the protests.
"Our lawyers can't keep up," he said.
Mohamed Lotfy, director of the Egyptian Commission for Rights and Freedoms, put the number at more than 450 and the Egyptian Center for Economic and Social Rights said 516 had been detained.
Sisi, a former army chief who came to power after the overthrow of President Mohamed Mursi, an Islamist, in 2013, has overseen a crackdown on political dissent that has targeted liberal activists as well as Islamists.
Sisi's supporters say the crackdown was necessary to stabilise Egypt after the 2011 uprising that toppled former strongman Hosni Mubarak.
The new unrest hit Egyptian financial markets. The EGX 30 and EGX 100 stock markets both fell by more than 5% on Sunday, their biggest single-day drops in several years.
Losses slowed on Monday, with the EGX 100 falling 1.72% and the EGX 30 dropping 1.47%. Egyptian dollar bonds issued by the government and the Egyptian pound in forward markets both slipped.
Demand for 10-year Egyptian bonds also dipped slightly.
The events over the weekend would "give cause for investors to re-price near-term political risks in Egypt", Farouk Soussa, senior economist with Goldman Sachs, said in research note.
"We think that the impact in external and domestic bond markets in the coming day or two may be more acute, given the high participation of international investors in these markets."
Security forces moved in to disperse the scattered protests on Friday and Saturday and have reinforced their presence in central Cairo.
Prominent human rights lawyer Mahienour El-Massry was arrested late on Sunday as she walked out of the state prosecutor's office in the capital city, where she was representing detainees, said Nour Fahmy, a lawyer with the Egyptian Commission for Rights and Freedoms.
"She was talking on the phone next to the national security prosecution, I passed by her ... then I heard her shouting 'I am being arrested' so I turned and saw three plainclothes police pushing her into a microbus and then they drove off," Fahmy said.
Plainclothes security officials have been stopping people in central Cairo and checking social media content on their mobile phones, several witnesses said.
Internet monitoring group NetBlocks said some social media platforms and news sites had been disrupted starting on Sunday. Arabic-language editions of international media which covered the protests appeared to be a target, said Alp Toker, executive director of NetBlocks.
Spokespeople at the interior and communications ministries could not immediately be reached for comment.
The protests took place after a former civilian contractor for the military, Mohamed Ali, posted videos accusing Sisi and the military of corruption. Sisi dismissed the claims as "lies and slander".
Ali has called for mass protests on Friday. (Reporting by Amina Ismail, Yousef Saba, Marc Jones and Tom Arnold, Writing by Aidan Lewis and Lena Masri, Editing by Ulf Laessing, Angus MacSwan and Sonya Hepinstall)