At least 26 civilians were killed in Cairo, and two others south of the capital, and 94 people were wounded, senior health ministry official Khaled al-Khatib told reporters.
In central Cairo, police have fired shots and tear gas to disperse stone-throwing protesters. AFP correspondents saw several suspected demonstrators being arrested and beaten.
"Home safe. No phone. Sorry for causing worry,'' Mr MacDiarmid tweeted a short while ago.
@jonamorem @basildabh @aporamsey Home now, thanks.
Mr MacDiarmid confirmed to APNZ he had been detained.
Earlier today, American photojournalist and Daily News Egypt reporter Aaron Rose said Mr MacDiarmid was arrested in Dokki, however no police stations had any information on the New Zealander.
"Kiwi journalist @CampbellMacD has been arrested in Dokki. We contacted NZ embassy and all our contacts. What next?'' Mr Rose wrote on Twitter.
A Ministry of Foreign Affairs and Trade spokeswoman said the NZ Embassy in Cairo received confirmation that Mr MacDiarmid is safe and well. "He was not arrested.''
Adam's girlfriend Amanda Mustard had earlier tweeted "National Security confirms that foreigners are in custody, one NZ citizen confirmed, presumably @CampbellMacD. Should know more after 10pm''.
When Ms Mustard spoke to APNZ she said, "presumably both Adam and Campbell have been taken''.
Ms Mustard said other freelance journalists had been trying to ascertain what had happened to them.
"We can't find anything out at this point. All the embassies are looking, a bunch of freelance Egyptian journalists are going to all the police stations to look, nothing has come out so far.''
Cairo-based journalist Bel Trew said on Twitter "My friend said a foreigner wearing a backpack, white t-shirt brown hair, tall being beaten badly right now - they're calling him a spy''.
"It appears we did witness beating of Kiwi journalist @CampbellMacD my friend tried to find out what happened to him but he'd been arrested.''
