US President Donald Trump and Indian Prime Minister Narendra Modi on Sunday declared themselves united in a relentless fight against "terrorism," vowing a close, personal alliance in front of tens of thousands of Indian-Americans.

The two leaders, like-minded nationalists fond of fiery rallies and skeptical of traditional media, heaped praise on each other in an unusual joint appearance inside a football stadium in Houston.

To the bhangra beats of four drummers in saffron turbans, Trump in his dark suit and Modi in a yellow kurta and vest made a grand entrance with arms clenched together to ecstatic cheers from a crowd estimated by organizers at 50,000.

Trump won his biggest applause when he told the crowd, many wearing the saffron of India's Hindu nationalist Bharatiya Janata Party, "We are committed to protecting innocent civilians from the threat of radical Islamic terrorism."

Taking the flavour of one of Trump's own boisterous rallies, Modi later asked the crowd to give a standing ovation to Trump for his stance.

Trump has stood by the Indian leader during controversial decisions this year, including his revocation of autonomy for Muslim-majority Kashmir and his order for jets to enter Pakistani territory in response to a suicide bombing.

With Trump watching in the front row and listening to the translation, Modi made clear reference to rival Pakistan, which controls part of Kashmir and has sought to rally international attention over the Himalayan territory.

Modi said he was seeking equal status and development for Kashmir, adding that his actions were "causing discomfort to some people unable to manage their own country" and who "nurture terrorism."

"These people have put their hatred of India at the center of their political agenda," Modi said.

India accuses Pakistan of arming Islamic militants who have fought its rule in Kashmir.

But India has also faced strong criticism from human rights activists for shutting down virtually all internet and cellular communications across much of Kashmir.

Protesters gathered outside of the NRG Stadium with placards and shirts that said, "Free Kashmir" and accused Modi of violating religious freedom -- a cause frequently evoked by the Trump administration.

Some Democrats take distance

The event -- dubbed, with a Texan twang, "Howdy, Modi!" -- was billed as the largest gathering ever by a foreign leader other than the pope in the United States.

Hoping to ensure that it remains bipartisan, organizers also invited prominent Democrats.

Steny Hoyer, the second-top Democrat in the House of Representatives, pledged that both major US parties wanted strong relations with India -- but gently voiced concern, pointing to India's historic "respect for secularism and human rights."

"Americans and Indians must strive to make our promises and aspirations a reality for all our citizens," he said with Modi at his side.

Presidential contender Bernie Sanders, who did not attend, was more direct, saying that Trump showed a "deafening silence" on the clampdown in Kashmir.

"I know that when a president stays silent in the face of religious persecution, repression and brutality, the dangerous message this sends to authoritarian leaders around the world is, 'Go ahead, you can get away with it,'" Sanders wrote in the Houston Chronicle.

Electoral prospects

Trump grinned broadly while Modi heaped praise on him, complementing him on his wit and even invoking the president's "Make America Great Again" slogan as he hailed the state of the US economy.

But Indian-Americans voted overwhelmingly for his rival Hillary Clinton in 2016.

Houston, one of the most ethnically diverse cites in the United States, is ground zero in the Democratic Party's inroads in Texas, a must-win state for Trump next year.

Speaking of his record as if on the campaign trail, Trump made no mention of many Indians' concerns over US visa policy -- but highlighted his efforts to turn back undocumented immigrants from Central America.

"We are going to take care of our Indian-American citizens before we take care of illegal immigrants that want to pour into our country," Trump said.

Hardly known for his celebrations of ethnic diversity, Trump said to Indian-Americans, "We love you."

"You enrich our culture, you uphold our values, you uplift our communities and you are truly proud to be American -- and we are proud to have you as Americans," he said.

Sporting a vest in yellow embroidery from Modi's home state of Gujarat as well as a cap in the Indian tricolor, Bhavin Parikh of Sacramento, California said he wanted to show support for Modi and called the event "historic" due to Trump's presence.

But he demurred on whether the gathering indicated backing Trump.

"It is not a question of Democrat or Republican. It's the American president supporting the Indian prime minister," he said.