CAIRO, Sept 20 (Reuters) - No sooner had Egyptian authorities painted over a wall of revolutionary graffiti near Tahrir Square this week than the street artists were back with spray cans and a new target: President Mohamed Mursi.

Seeking to restore a sense of normalcy to Tahrir, scene of the democratic uprising that swept Hosni Mubarak from power last year, the authorities have deployed police, evicted unlicensed vendors and planted palm trees, shrubs and flowers.

But the move to whitewash graffiti charting the course of the revolt and the turbulent 18 months that followed was a step too far for the artists. They congregated on Wednesday and Thursday to spray murals expressing anger with the government.

"This work embodied many things: the martyrs, the military regime and a people looking for freedom and democracy," said Ahmed Nadi, a political cartoonist, as he spray-painted caricatures of the bearded, bespectacled president who was elected in June in Egypt's first free presidential vote.

"It was the memory of a place that witnessed many important events," he said. "We imagined that Mursi would leave these great images as they were."

Though Mursi apparently had nothing to do with the step - officials said the clean-up had been ordered by the governor of Cairo - the artists and activists who gathered to support them held the newly-elected Islamist leader responsible.

Prime Minister Hisham Kandil issued a statement distancing the government from a move he attributed to workers from the governorate of Cairo, hinting at the disconnect between the executive and the sprawling state. He called for more graffiti that "reflects the spirit of the Jan. 25 revolution".

Street art has been a defining feature of the Egyptian revolution and the new freedoms it brought. Photographed widely by tourists and Egyptians alike, the murals have taken aim at Mubarak and the military rulers who replaced him, and paid tribute to activists killed during the uprising.

"WE WILL PAINT IT AGAIN!"

During a spell of army rule that ended with Mursi's election, the authorities painted over the murals several times. On each occasion, the artists returned to reassert what they see as part of their right to freedom of speech.

The governorate of Cairo has not said why the murals were effaced this time. The spokesman for the authority could not be reached for comment.

Admirers of the work have cited its humour and artistic qualities: some of the murals have incorporated elements of ancient pharaonic art.

But the furore surrounding their removal has triggered a degree of debate about the rights and wrongs of street art in a country where Islamists are now at the heart of public life.

Some conservative Islamists believe such images are forbidden in Islam.

"If it is acceptable for a revolutionary youth to draw on the walls of a public street, what is to stop another person from clearing these murals on the grounds that he believes, for example, that depicting people is forbidden?" Hossam Abdel Aziz, a conservative Islamist writer, wrote on his Facebook page.

The graffiti artists are typically leftists, liberals or communists. Nadi, the artist who was drawing Mursi, described his own views as a mix of leftism and Islamism.

"Wipe it away and we will paint it again!" chanted a group of several dozen protesters who watched as the artists set about their work, using charcoal to sketch out the new murals on the wall in Mohamed Mahmoud Street, a theatre for street battles between protesters and the security forces this year and last.

"We removed Mubarak to get our freedom. Nobody will come and take our freedom. We will paint!" shouted 28-year old sculptor Hany Rashed as he joined the demonstration."

"Until the day I die, nobody will stop me."

At least seven truckloads of riot police were stationed along the road, but they did not intervene. Some of the policemen smiled as they watched the artists at work. Shortly afterwards, their trucks left the area altogether.

The police moved into Tahrir in large numbers on Saturday, evicting unlicensed street vendors and destroying a handful of tents. The step followed the end of demonstrations at the nearby U.S. Embassy over a film that denigrated the Prophet Mohammad.

Traffic police have also redeployed in the square in large numbers for the first time since the uprising. Tahrir is one of the busiest traffic hubs in the vast capital and is bordered by the Egyptian Museum, the Arab League headquarters and the Mugamma - one of the state's biggest bureaucratic institutions.

Gardeners watered freshly laid grass and planted flowers in the traffic circle in the middle of the square on Thursday.

Mohamed al-Lashimi, who runs a newspaper stand operated by his family for 45 years, said the clean-up should have come sooner. "The people were afraid to come down here - afraid of the thugs," said the 35-year old, attributing increased sales at his stall this week to the security presence.

Abdel Hamid Ibrahim, 65, watched the gardeners watering the flowers and grass. "I am happy to see this. I hope it stays like this and gets even better," he said. "We want to move forward. We cannot move back." (Additional reporting by Yasmine Saleh and Tamim Elyan; Editing by Mark Heinrich)