The agreement had brought Nepal close to stability as Maoist fighters joined an interim parliament and began to put down their weapons.
Upendra Yadav, 48, the forum's chairman, said: "We will continue the struggle if the government ignores our demands, and the future of the country will be difficult."
The arrest of Yadav this month, for burning a copy of the interim constitution, was one of the incidents that sparked protests in the southeastern town of Lahan, which is still under curfew after becoming the focus of unrest.
The protests escalated after a Maoist fighter shot dead a 16-year-old boy last week.
Police said they had to open fire when Madhesi protesters tried to storm a police station a few days later. Four demonstrators died and scores were wounded.
Nepal's Terai region, also known as Madhesh, is home to nearly half of the country's 26 million people and many residents have deeper cultural and linguistic links with the people of India than with Nepalis living in the mountains.
Covering 23 per cent of landlocked Nepal, it is the country's bread basket, providing maize, rice and wheat.
Yadav, a college teacher from Sunsari district, said: "They [the politicians and Maoists] have never thought of us as one of their own. They take our votes and revenues and that is all."
On Wednesday, in the latest violence, at least 20 people were hurt in clashes between Madhesi activists and Maoists in the town of Janakpur near the Indian border.
Madhesis then torched a Maoist office and a police post, forcing authorities to instigate a night curfew.
Yadav, who was set free after protests demanding his release, said: "If we did not shout and protest, no one would have heard of us."
One Nepalese magazine editor said it was a mistake for the Maoists and the ruling alliance to agree to an interim constitution without addressing the concerns of the people in the Terai.
Yubaraj Ghimire, editor of Samay, said: "The government and Maoists have mishandled the whole thing. They have been outrightly dismissive of Madhesi worries.
"If they continue with such a dismissive attitude, this could escalate and undermine the peace process."
Madhesi activists say that although the people in the Terai constitute nearly half the population, less than a quarter of the interim parliament's 330 members come from the region.
Yadav said: "We want respect and autonomy and proportional representation in the political, administrative and all decision-making processes of the country."