KUALA LUMPUR (Reuters) – Would-be arsonists in mostly Muslim Malaysia struck at a convent school and a fifth church on Sunday while church leaders called for calm in a row over Christians' use of the word "Allah" to refer to God.

The attacks threaten Prime Minister Najib Razak's plan to win back non-Muslim support before elections due by 2013 and may scare investors away from Malaysia that has trailed Thailand and Indonesia for foreign investment.

Police in the sleepy city of Taiping, around 300 km (185 miles) from the capital Kuala Lumpur, said a petrol bomb was thrown at the guard house of a Catholic convent school but failed to go off.

They also said they had found several broken bottles including paint thinners outside one of the country's oldest Anglican churches, All Saints, Taiping, and said one of the building's walls had been blackened.

The row, over a court ruling that allowed a Catholic newspaper to use "Allah" in its Malay-language editions, had prompted Muslims to protest at mosques and sparked arson attacks on four churches that saw one Pentecostalist church gutted.

On Sunday, Malaysians packed churches to listen to sermons of "reaching out in friendship to all, including Muslims" and "keeping the peace in multi-religious Malaysia" but many felt their religious rights were being trampled.

"There are extremists in this country and the government seems unable to do anything," said Wilson Matayun, a salesman who attended Mass at St Anthony's Church in Kuala Lumpur. "I am losing faith in our government. I pray it does not get worse."

Matayun is from Sabah state on Borneo island, where a large number of non-English speaking Christians have used the word "Allah" for decades. Christians account for 9.1 percent of the 28 million population.

The government has appealed against the ruling, a marked contrast to countries like Indonesia, Egypt and Syria where Christian minorities freely use the Arabic word to refer to God.

Malaysia is mainly Muslim and Malay but there are sizable ethnic Chinese and Indian communities who mainly practice Christianity, Buddhism and Hinduism.

They handed the government its biggest losses in 2008 state and national elections in part due to feelings of religious marginalization and growing disillusionment with corruption.

Najib's handling of the issue will determine whether he can keep the support of the Malays and win back ethnic Chinese and Indian voters to solidify his grip on power after taking control of the government last year.