By Roger Hardy BBC Middle East analyst in Riyadh

Reports say police arrested dozens of protesters

While officials have referred to it dismissively as an illegal gathering, ordinary Saudis see it as another sign of the growing pressure for change in the traditionally conservative kingdom.

Wednesday's Saudi newspapers quoted the interior minister, Prince Nayef, as saying there were no more than 150 protesters at the demonstrations in the heart of Riyadh, though eyewitnesses told the BBC there may have been as many as 500.

The newspapers described the protest as an illegal gathering - which technically it was.

But conversations with ordinary Saudis suggest the event is sending shock waves through a society that is unaccustomed to large-scale protests.

Pace of reform

The event has embarrassed the Saudi authorities because it took place in the heart of the capital and was in response to a call from a London-based opposition group.

There is great pressure on the Gulf monarchy to reform

This in itself shows that nowadays the internet, satellite TV and the mobile phone can be powerful political tools.

And the demonstration took place at the very moment when the government had just announced that the country's first elections would be held within a year and was holding its first conference on human rights.

The official line seems to be that the government remains firmly committed to both political and economic reform, but that it alone should determine the pace and content of change.

However, Tuesday's protest suggests the road to reform may not be smooth. There is a palpable sense of unease here, directly attributable to the series of suicide bombings which shook Riyadh in May.

This has not only heightened the sense of insecurity, but raised troubling questions about the country's future.