Built in the 12th century under Norman rule, it boasts Italy's largest Byzantine-style mosaics, second in the world only to those of Hagia Sophia in Istanbul.

While it's not possible to rule this out, the BBC has seen three separate videos filmed by the fighters themselves, in which RSF insignia can be seen on the uniforms of those involved.

But it said: "China is a country governed by the rule of law, and strictly safeguards citizens' rights to lawfully express their concerns and provide opinions or suggestions."

The same is true in civil society — when the rules are violated, the whole system is weakened.

"The golden rule: treat others as you'd like to be treated," Ms Windsor adds.