At least 18 people have been killed in clashes between police and protesters across Egypt, officials said.

Three police cadets were among the dead, and dozens of protesters were also injured, the officials said.

The clashes follow the death of an activist in a march in the capital Cairo on Saturday.

The protests were staged to mark the fourth anniversary of Egypt's 2011 uprising, which toppled long-time leader Hosni Mubarak.

Security in major cities was tightened ahead of the anniversary, and key locations in Cairo were blocked off.

Dozens of people were killed in similar protests last year.

Crackdown on dissent

The police cadets were killed during protests in Cairo, the officials said, while protesters were also killed in the capital and the northern city of Alexandria.

Police said two militants were killed as they tried to plant an explosive device in the Nile Delta.

More than 400 people were arrested.

On Saturday, activist Shaimaa al-Sabbagh, aged 32, was shot dead during a march by the socialist Popular Alliance party in central Cairo.

The party blamed police for her death. She was buried amid angry scenes in Alexandria on Sunday. Hundreds of people attended the funeral.

Prosecutors have launched an investigation into her death.

Since ousting the elected President Mohammed Morsi in July 2013, Egypt under new leader Abdel Fattah al-Sisi has cracked down on political dissent.

Source: BBC