That states should not prevent any peaceful protest in public

A gathering of people who are staging a public protest normally against an action, policy or ideal. This protest is non violent and should not present any danger to the general public or be used as a platform to incite violence. A few examples of successful peaceful protests are: Ghandi's Salt March, the Montgomery Bus Boycott, the Singing Revolution and the Tree Sitters of Pureora. Whether the protest is successful or not is independent of the right for those protests to exist but it is good to show that they can be an effective policy even against large problems like institutional racism. 

In many cases in westernized democracies there is some necessity to inform the police of the protest if this will involve a march so they can direct traffic and keep the march from causing problems for other citizens and in general keep the peace.

Getting media attention and showing popular support are the main benefits for the protesters themselves. This is self explanatory in that the media will report on a large gathering of people or an unusual form of protesting which allows more people to find out about the cause and show that there is some sort of public backing for the cause already. This means that more people who feel strongly about the issue are likely to take up protesting as well and those who did not know the issue existed have a bit more understanding of it. All of this helps achieve their ultimate goal of solving the issue normally though policy. When they can show enough people support the cause and can make the issue a constant talking point in the media then politicians are forced to give some sort of response.

Peaceful protests actually benefit the general populous by acting as a check and balance for the government making sure that the government is still upholding the will of the people. One example of this is the protests against the Vietnam war. I obviously only support the peaceful protests that took place (which is the majority). The Vietnam War was an excellent example of the government abusing the power appointed to them by the people to engage in a war that was unnecessary and went against the best interests of the American people due to how many were killed, injured and maimed. The protests were sparked by people finding out that the government had mislead the populous about the state of the war and the reasons for that war. The protests were then a large part of that war ending and showing others internationally that their governments actions did not represent them.

The main protection is that it is considered a human right under the Humans Rights Acts Article 11. There are normally legal requirements for the safe and legal implementations of this right so that it negatively impacts other citizens as little as possible. This is seen in the example I provided for marches in the UK and can be seen in this document from Australia about the legal requirements of peaceful protests. 

States have an immense amount of power over their populous because they set the laws and have a monopoly over violence which can be used to enforce the laws and have power over institutes such as media which can be used to suppress the people. This is seen in countries such as North Korea, China and Russia. Peaceful protests are one of the few means of populous control over government actions outside of voting. This is important because not all issues can be accounted for during an election and they're has to be some means of holding a government accountable to the promises they were elected on. Governments exist to protect the interests of the populous and provide some sort of protections. Governments that do not comply with this are normally aggressive dictatorships (Syrian Assad regime) where the sole propose of that government is to gain power for the individuals in charge. This is an immoral suppression of people and is not an effective form of governance (see the violent uprising in Syria). States are obligated to provide the freedom of speech to people and by extension the right to peaceful protests otherwise this is an abuse of the power they have. 

In conclusion it is immoral for states to prevent these protests and they are a necessary part of continual governance which can achieve the they set out.