* Detained men helped attackers with mobile phone cards
* Pakistan went on alert after hoax call-report
* Indian airports on alert for fourth day
* Explosives found in hospital
By Tamajit Pain
KOLKATA, Dec 6 Indian police said on Saturday they had arrested two men who helped the Mumbai attackers get mobile phone cards which they used for communications during their three-day rampage.
Police in the eastern city of Kolkata identified the men as Tausif Rehman and Mukhtar Ahmed and said they were picked up on Friday after investigators traced some of the Subscriber Identity Module (SIM) cards recovered from the gunmen.
"We are questioning them about procurement of SIM cards used in Mumbai," Jawed Shamim, deputy commissioner of detectives in Kolkata, told Reuters.
The arrests are further evidence of Indian complicity in the three-day rampage. New Delhi has blamed the attacks on Islamic militants from neighbouring Pakistan, raising tension between South Asia's longtime foes, both nuclear-armed.
Airports in New Delhi, Bangalore and Chennai remained on high alert for a fourth day on Saturday, with extra security personnel deployed after India's civil aviation authority said it had received intelligence that attacks could be planned.
Security was also high in the north Indian town of Ayodhya on Saturday, the 16th anniversary of the razing of the Babri mosque by a Hindu extremist mob which set off Hindu-Muslim riots that killed thousands.A makeshift Hindu temple now stands there.
Hindu nationalists burned five Pakistani flags in front of the temple site on Saturday to shouts of "Down with Pakistan!"
"We decided to also condemn the terror unleashed by Pakistan on our country," a spokesman for the group, Sharad Sharma, said. "And what could have been a better way to condemn than setting ablaze the flags of that country?"
At least 700 extra officers were protecting the site, district police chief R.K.S. Rathore told Reuters.
Earlier, 15 Hindu activists demanding a permanent temple be built were arrested in nearby Faizabad while Muslim activists who ordinarily fly a black flag on the anniversary opted not to.
"We are not making any public protest this time in view of the large-scale carnage by terrorists in Mumbai," activist leader Yunus Siddiqui told Reuters.
At least 171 people were killed in the attacks last week in which 10 gunmen struck two luxury hotels and other landmarks in India's financial capital.
India has remained jittery since the attacks, and on Saturday police in Nagpur in the western state of Maharashtra said they had found explosives in a hospital after a caller phoned doctors with a warning from a public phone booth.
Aziz Khan, a doctor at Crescent Hospital, said the caller, speaking Hindi, said a bomb had been placed near the hospital entrance: "You will see the result in 10 minutes," the caller told him.
Patients were evacuated and a police bomb squad and sniffer dog were called in. They found some explosives, Nagpur Joint Commissioner of Police Babasaheb Kangale said.
Nagpur is home to Rashtriya Swayamsevak Sangh, a Hindu nationalist group that supports the main opposition Bharatiya Janata Party. The BJP has accused the ruling Congress party coalition of being weak on security.
Police said they were pursuing details of local Indian help for the Mumbai attackers after the arrests of Rehman and Ahmed.
Rehman, a clerk, used a dead relative's identity documents to acquire the 22 SIM cards which he later sold to Ahmed, Shamim said later. Both men were charged with conspiracy and forgery.
Ahmed was a street vendor and three-wheel taxi driver in Kolkata, Shamim said. He was arrested in New Delhi.
Shamim said it was not immediately clear how the SIM cards were passed to the gunmen, whom investigators have said talked to their handlers during the 60-hour rampage.
Mumbai police have said the gunmen were controlled by the Pakistan-based Lashkar-e-Taiba (LeT) group blamed for earlier attacks in India, including a 2001 assault on India's parliament that very nearly sparked a war between India and Pakistan.
LeT is on U.S. and Indian terrorist lists and Indian police say two of its operations leaders, designated terrorists by Washington in May, coordinated the Mumbai rampage.
There has been public anger at intelligence failures in preventing the attacks. India's newly appointed home minister admitted on Friday there had been lapses.
Pakistan's Dawn newspaper reported on Saturday that Pakistan had put its forces on high alert after a hoax caller pretending to be India's foreign minister spoke threateningly to President Asif Ali Zardari two days after the attacks. [nISL383109] (Reporting by New Delhi, Mumbai and Islamabad bureaux; Additional reporting by Sharat Pradhan in Ayodhya; Writing by Bryson Hull; editing by Tim Pearce)