Tens of thousands of people packed Cairo's Tahrir Square on Friday, after the first call by Islamist leaders for nationwide demonstrations since President Hosni Mubarak was overthrown in February. Many protesters — dominated by Muslim Brotherhood supporters — are calling for an Islamic state and Sharia law. The Brotherhood is the most organized political force in Egypt, although it was not prominent in the revolution. Tensions have been running high between Egypt's Islamist and secular groups, who are at odds over the transition to democracy in the Arab world's most populated country. The World's Matthew Bell is in Cairo.