"Police fired live rounds at protesters, killing at least one of them and wounding five," the organising committee member told AFP news agency on Tuesday.
Other Houthi sources told Al Jazeera that at least five activists were killed.
Al Jazeera's Hashem Ahelbarra, reporting from Sanaa, said there was a "serious escalation" of under way in Sanaa.
"More than 50 people were also injured in the clashes," he said.
The activists have been camped out in Sanaa for weeks in a campaign to bring down the government and reverse unpopular fuel price increases.
Fighting has also intensified in the northern al-Jawf province where the government is conducting air strikes against Houthi positions.
The authorities claim a senior rebel commander was killed in one of these raids.